Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 5:51 AM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Wed, Aug 19, 2015 at 05:23:41AM -0700, H.J. Lu wrote: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Yes, we want to use the argument pointer as shown in testcases included in my patch. Where do we stand on this? We need the hard stack address at function entry for x86 without using frame pointer. I added __builtin_stack_top since __builtin_frame_address can't give us what we want. Should __builtin_stack_top be added to middle-end or x86 backend? Sorry for not following up; I thought my suggestion was obvious. Can you do a __builtin_argument_pointer instead? That should work for all targets, afaics? To me, stack top is easier to understand and argument pointer isn't very clear. Does argument pointer exist when there is no argument? But I can live with it. I will update my patch. Thanks. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 1:50 PM, H.J. Lu hjl.to...@gmail.com wrote: On Tue, Aug 4, 2015 at 1:45 PM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Tue, Aug 04, 2015 at 01:00:32PM -0700, H.J. Lu wrote: There is another issue with x86, maybe other targets. You can't get the real stack top when stack is realigned and -maccumulate-outgoing-args isn't used since ix86_expand_prologue will create and return another stack frame for __builtin_frame_address and __builtin_return_address. It will be wrong for __builtin_stack_top, which should return the real stack address. That's why I asked: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Yes, we want to use the argument pointer as shown in testcases included in my patch. Where do we stand on this? We need the hard stack address at function entry for x86 without using frame pointer. I added __builtin_stack_top since __builtin_frame_address can't give us what we want. Should __builtin_stack_top be added to middle-end or x86 backend? Thanks. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 05:23:41AM -0700, H.J. Lu wrote: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Yes, we want to use the argument pointer as shown in testcases included in my patch. Where do we stand on this? We need the hard stack address at function entry for x86 without using frame pointer. I added __builtin_stack_top since __builtin_frame_address can't give us what we want. Should __builtin_stack_top be added to middle-end or x86 backend? Sorry for not following up; I thought my suggestion was obvious. Can you do a __builtin_argument_pointer instead? That should work for all targets, afaics? Segher
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 6:00 AM, H.J. Lu hjl.to...@gmail.com wrote: On Wed, Aug 19, 2015 at 5:51 AM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Wed, Aug 19, 2015 at 05:23:41AM -0700, H.J. Lu wrote: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Yes, we want to use the argument pointer as shown in testcases included in my patch. Where do we stand on this? We need the hard stack address at function entry for x86 without using frame pointer. I added __builtin_stack_top since __builtin_frame_address can't give us what we want. Should __builtin_stack_top be added to middle-end or x86 backend? Sorry for not following up; I thought my suggestion was obvious. Can you do a __builtin_argument_pointer instead? That should work for all targets, afaics? To me, stack top is easier to understand and argument pointer isn't very clear. Does argument pointer exist when there is no argument? But I can live with it. I will update my patch. Here is a patch to add __builtin_argument_pointer. I only have -- Built-in Function: void * __builtin_argument_pointer (void) This function returns the argument pointer. as documentation. Can you suggest a better description so that it can be implemented also by other compilers? Thanks. -- H.J. From 9af08fdda587e1876e09840499000e35cc841e96 Mon Sep 17 00:00:00 2001 From: H.J. Lu hjl.to...@gmail.com Date: Tue, 21 Jul 2015 14:32:09 -0700 Subject: [PATCH] Add __builtin_argument_pointer When __builtin_frame_address is used to retrieve the address of the function stack frame, the frame pointer register is required, which wastes one register and 2 instructions. For x86-32, one less register means significant negative impact on performance. This patch adds a new builtin function, __builtin_argument_pointer. It returns the argument pointer, which, on x86, can be used to compute the stack address when the function is called by subtracting the size of integer register. gcc/ PR target/66960 * builtin-types.def (BT_FN_PTR_VOID): New function type. * builtins.c (expand_builtin): Handle BUILT_IN_ARGUMENT_POINTER. (is_simple_builtin): Likewise. * ipa-pure-const.c (special_builtin_state): Likewise. * builtins.def: Add BUILT_IN_ARGUMENT_POINTER. * function.h (function): Add argument_pointer_taken. * config/i386/i386.c (ix86_expand_prologue): Sorry if DRAP is used and the argument pointer has been taken. * doc/extend.texi: Document __builtin_argument_pointer. gcc/testsuite/ PR target/66960 * gcc.target/i386/pr66960-1.c: New test. * gcc.target/i386/pr66960-2.c: Likewise. * gcc.target/i386/pr66960-3.c: Likewise. * gcc.target/i386/pr66960-4.c: Likewise. * gcc.target/i386/pr66960-5.c: Likewise. --- gcc/builtin-types.def | 1 + gcc/builtins.c| 5 + gcc/builtins.def | 1 + gcc/config/i386/i386.c| 6 ++ gcc/doc/extend.texi | 4 gcc/function.h| 3 +++ gcc/ipa-pure-const.c | 1 + gcc/testsuite/gcc.target/i386/pr66960-1.c | 34 +++ gcc/testsuite/gcc.target/i386/pr66960-2.c | 34 +++ gcc/testsuite/gcc.target/i386/pr66960-3.c | 18 gcc/testsuite/gcc.target/i386/pr66960-4.c | 22 gcc/testsuite/gcc.target/i386/pr66960-5.c | 22 12 files changed, 151 insertions(+) create mode 100644 gcc/testsuite/gcc.target/i386/pr66960-1.c create mode 100644 gcc/testsuite/gcc.target/i386/pr66960-2.c create mode 100644 gcc/testsuite/gcc.target/i386/pr66960-3.c create mode 100644 gcc/testsuite/gcc.target/i386/pr66960-4.c create mode 100644 gcc/testsuite/gcc.target/i386/pr66960-5.c diff --git a/gcc/builtin-types.def b/gcc/builtin-types.def index 0e34531..2b6b5ab 100644 --- a/gcc/builtin-types.def +++ b/gcc/builtin-types.def @@ -177,6 +177,7 @@ DEF_FUNCTION_TYPE_1 (BT_FN_COMPLEX_LONGDOUBLE_LONGDOUBLE, BT_COMPLEX_LONGDOUBLE, BT_LONGDOUBLE) DEF_FUNCTION_TYPE_1 (BT_FN_PTR_UINT, BT_PTR, BT_UINT) DEF_FUNCTION_TYPE_1 (BT_FN_PTR_SIZE, BT_PTR, BT_SIZE) +DEF_FUNCTION_TYPE_1 (BT_FN_PTR_VOID, BT_PTR, BT_VOID) DEF_FUNCTION_TYPE_1 (BT_FN_INT_INT, BT_INT, BT_INT) DEF_FUNCTION_TYPE_1 (BT_FN_INT_UINT, BT_INT, BT_UINT) DEF_FUNCTION_TYPE_1 (BT_FN_INT_LONG, BT_INT, BT_LONG) diff --git a/gcc/builtins.c b/gcc/builtins.c index 31969ca..b1cfa44 100644 --- a/gcc/builtins.c +++ b/gcc/builtins.c @@ -6206,6 +6206,10 @@ expand_builtin (tree exp, rtx target, rtx subtarget, machine_mode mode, case BUILT_IN_CONSTANT_P: return
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 9:58 AM, Segher Boessenkool
seg...@kernel.crashing.org wrote:
On Wed, Aug 19, 2015 at 08:25:49AM -0700, H.J. Lu wrote:
Here is a patch to add __builtin_argument_pointer. I only have
Sorry to be a pain but... all the other builtins use _address
instead of _pointer, it's probably best to follow that.
-- Built-in Function: void * __builtin_argument_pointer (void)
This function returns the argument pointer.
as documentation. Can you suggest a better description so that it can
be implemented also by other compilers?
Maybe something like (heavily cut'n'pasted):
@deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
This doesn't make senses when there is no argument or arguments
are passed in registers. To me, argument pointer is a virtual concept
and an implementation detail internal to GCC. I am not sure if another
compiler can implement it based on this description.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the first
argument that is on the stack.
+ /* Can't use DRAP if the stack address has been taken. */
+ if (cfun-argument_pointer_taken)
+ sorry (%__builtin_argument_pointer% not supported with stack
+ realignment. This may be worked around by adding
+ -maccumulate-outgoing-args.);
This doesn't work with DRAP? Pity :-(
With DRAP, we do
/* Replicate the return address on the stack so that return
address can be reached via (argp - 1) slot. This is needed
to implement macro RETURN_ADDR_RTX and intrinsic function
expand_builtin_return_addr etc. */
t = plus_constant (Pmode, crtl-drap_reg, -UNITS_PER_WORD);
t = gen_frame_mem (word_mode, t);
insn = emit_insn (gen_push (t));
RTX_FRAME_RELATED_P (insn) = 1;
/* For the purposes of frame and register save area addressing,
we've started over with a new frame. */
m-fs.sp_offset = INCOMING_FRAME_SP_OFFSET;
m-fs.realigned = true;
which doesn't work for __builtin_argument_pointer.
The patch looks plausible, but I of course can not approve it.
Thanks.
--
H.J.
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 10:08:01AM -0700, H.J. Lu wrote:
Maybe something like (heavily cut'n'pasted):
@deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
This doesn't make senses when there is no argument or arguments
are passed in registers.
Sure, but see the weasel-words below (The exact...)
To me, argument pointer is a virtual concept
and an implementation detail internal to GCC. I am not sure if another
compiler can implement it based on this description.
The same is true for frame_address, on many machines.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the first
argument that is on the stack.
Segher
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 10:48 AM, Segher Boessenkool
seg...@kernel.crashing.org wrote:
On Wed, Aug 19, 2015 at 10:08:01AM -0700, H.J. Lu wrote:
Maybe something like (heavily cut'n'pasted):
@deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
This doesn't make senses when there is no argument or arguments
are passed in registers.
Sure, but see the weasel-words below (The exact...)
To me, argument pointer is a virtual concept
and an implementation detail internal to GCC. I am not sure if another
compiler can implement it based on this description.
The same is true for frame_address, on many machines.
Stack frame is well understood unlike argument pointer which is
pretty vague.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the first
argument that is on the stack.
Segher
--
H.J.
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 08:25:49AM -0700, H.J. Lu wrote:
Here is a patch to add __builtin_argument_pointer. I only have
Sorry to be a pain but... all the other builtins use _address
instead of _pointer, it's probably best to follow that.
-- Built-in Function: void * __builtin_argument_pointer (void)
This function returns the argument pointer.
as documentation. Can you suggest a better description so that it can
be implemented also by other compilers?
Maybe something like (heavily cut'n'pasted):
@deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the first
argument that is on the stack.
+ /* Can't use DRAP if the stack address has been taken. */
+ if (cfun-argument_pointer_taken)
+ sorry (%__builtin_argument_pointer% not supported with stack
+ realignment. This may be worked around by adding
+ -maccumulate-outgoing-args.);
This doesn't work with DRAP? Pity :-(
The patch looks plausible, but I of course can not approve it.
Thanks,
Segher
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 03:18:46PM -0700, H.J. Lu wrote:
@deftypefn {Built-in Function} {void *} __builtin_argument_pointer (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the
first argument which would be passed on the stack.
@end deftypefn
That is fine by me. Thanks!
Segher
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 3:10 PM, Segher Boessenkool
seg...@kernel.crashing.org wrote:
On Wed, Aug 19, 2015 at 02:53:47PM -0700, H.J. Lu wrote:
How about this
@deftypefn {Built-in Function} {void *} __builtin_argument_pointer (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame. Unlike
@code{__builtin_frame_address}, the frame pointer register isn't
required.
That last line isn't true, if your port uses INITIAL_FRAME_POINTER_RTX.
Maybe it shouldn't be true otherwise either (but currently a hard frame
pointer is forced, indeed). Have we gone full circle now? ;-)
Let's drop it:
@deftypefn {Built-in Function} {void *} __builtin_argument_pointer (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the
first argument which would be passed on the stack.
@end deftypefn
--
H.J.
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 10:53 AM, H.J. Lu hjl.to...@gmail.com wrote:
On Wed, Aug 19, 2015 at 10:48 AM, Segher Boessenkool
seg...@kernel.crashing.org wrote:
On Wed, Aug 19, 2015 at 10:08:01AM -0700, H.J. Lu wrote:
Maybe something like (heavily cut'n'pasted):
@deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame.
This doesn't make senses when there is no argument or arguments
are passed in registers.
Sure, but see the weasel-words below (The exact...)
To me, argument pointer is a virtual concept
and an implementation detail internal to GCC. I am not sure if another
compiler can implement it based on this description.
The same is true for frame_address, on many machines.
Stack frame is well understood unlike argument pointer which is
pretty vague.
How about this
@deftypefn {Built-in Function} {void *} __builtin_argument_pointer (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame. Unlike
@code{__builtin_frame_address}, the frame pointer register isn't
required.
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the
first argument which would be passed on the stack.
@end deftypefn
--
H.J.
Re: [PATCH] Add __builtin_stack_top
On Wed, Aug 19, 2015 at 02:53:47PM -0700, H.J. Lu wrote:
How about this
@deftypefn {Built-in Function} {void *} __builtin_argument_pointer (void)
This function is similar to @code{__builtin_frame_address} with an
argument of 0, but it returns the address of the incoming arguments to
the current function rather than the address of its frame. Unlike
@code{__builtin_frame_address}, the frame pointer register isn't
required.
That last line isn't true, if your port uses INITIAL_FRAME_POINTER_RTX.
Maybe it shouldn't be true otherwise either (but currently a hard frame
pointer is forced, indeed). Have we gone full circle now? ;-)
The exact definition of this address depends upon the processor and the
calling convention. Usually some arguments are passed in registers and
the rest on the stack, and this builtin returns the address of the
first argument which would be passed on the stack.
@end deftypefn
Segher
Re: [PATCH] Add __builtin_stack_top
On Aug 4, 2015, at 5:30 AM, H.J. Lu hjl.to...@gmail.com wrote: Where does this feature belong? I prefer the middle end.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 8:40 AM, Mike Stump mikest...@comcast.net wrote: On Aug 4, 2015, at 5:30 AM, H.J. Lu hjl.to...@gmail.com wrote: Where does this feature belong? I prefer the middle end. Any comments on my middle-end patch? Thanks. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 10:43 AM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Tue, Aug 04, 2015 at 10:28:00AM -0700, H.J. Lu wrote: Any comments on my middle-end patch? So, if the answer is the same as frame_address (0), why not have the fallback just expand to that? Then, one can use this builtin everywhere that frame address is used today. People that want a faster, tighter port can then implement the hook and achieve higher performance. The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, you don't get crtl-accesses_prior_frames set either, and as far as I can see everything works fine? For __builtin_frame_address(0). You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. expand_builtin_return_addr sets crtl-accesses_prior_frames = 1; for __builtin_frame_address, which requires a frame pointer register. __builtin_stack_top doesn't set crtl-accesses_prior_frames and frame pointer register isn't required. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Aug 4, 2015, at 8:44 AM, H.J. Lu hjl.to...@gmail.com wrote: On Tue, Aug 4, 2015 at 8:40 AM, Mike Stump mikest...@comcast.net wrote: On Aug 4, 2015, at 5:30 AM, H.J. Lu hjl.to...@gmail.com wrote: Where does this feature belong? I prefer the middle end. Any comments on my middle-end patch? So, if the answer is the same as frame_address (0), why not have the fallback just expand to that? Then, one can use this builtin everywhere that frame address is used today. People that want a faster, tighter port can then implement the hook and achieve higher performance.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 10:16 AM, Mike Stump mikest...@comcast.net wrote: On Aug 4, 2015, at 8:44 AM, H.J. Lu hjl.to...@gmail.com wrote: On Tue, Aug 4, 2015 at 8:40 AM, Mike Stump mikest...@comcast.net wrote: On Aug 4, 2015, at 5:30 AM, H.J. Lu hjl.to...@gmail.com wrote: Where does this feature belong? I prefer the middle end. Any comments on my middle-end patch? So, if the answer is the same as frame_address (0), why not have the fallback just expand to that? Then, one can use this builtin everywhere that frame address is used today. People that want a faster, tighter port can then implement the hook and achieve higher performance. The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 04, 2015 at 10:28:00AM -0700, H.J. Lu wrote: Any comments on my middle-end patch? So, if the answer is the same as frame_address (0), why not have the fallback just expand to that? Then, one can use this builtin everywhere that frame address is used today. People that want a faster, tighter port can then implement the hook and achieve higher performance. The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, you don't get crtl-accesses_prior_frames set either, and as far as I can see everything works fine? For __builtin_frame_address(0). You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. Segher
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 11:50 AM, H.J. Lu hjl.to...@gmail.com wrote: On Tue, Aug 4, 2015 at 10:43 AM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Tue, Aug 04, 2015 at 10:28:00AM -0700, H.J. Lu wrote: Any comments on my middle-end patch? So, if the answer is the same as frame_address (0), why not have the fallback just expand to that? Then, one can use this builtin everywhere that frame address is used today. People that want a faster, tighter port can then implement the hook and achieve higher performance. The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, you don't get crtl-accesses_prior_frames set either, and as far as I can see everything works fine? For __builtin_frame_address(0). You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. expand_builtin_return_addr sets crtl-accesses_prior_frames = 1; for __builtin_frame_address, which requires a frame pointer register. __builtin_stack_top doesn't set crtl-accesses_prior_frames and frame pointer register isn't required. BTW, x86 doesn't define INITIAL_FRAME_ADDRESS_RTX. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 12:29 PM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Tue, Aug 04, 2015 at 11:50:00AM -0700, H.J. Lu wrote: The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, you don't get crtl-accesses_prior_frames set either, and as far as I can see everything works fine? For __builtin_frame_address(0). You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. expand_builtin_return_addr sets crtl-accesses_prior_frames = 1; for __builtin_frame_address, which requires a frame pointer register. __builtin_stack_top doesn't set crtl-accesses_prior_frames and frame pointer register isn't required. Not if you have INITIAL_FRAME_ADDRESS_RTX. I don't see why the generic code cannot just use frame_pointer_rtx (instead of hard_frame_pointer_rtx) for a count of 0; but making it target-specific is certainly more conservative. You say i386 doesn't have that target macro defined currently. Yes I know; so change that? Or change the generic code, but that is much more testing. There is another issue with x86, maybe other targets. You can't get the real stack top when stack is realigned and -maccumulate-outgoing-args isn't used since ix86_expand_prologue will create and return another stack frame for __builtin_frame_address and __builtin_return_address. It will be wrong for __builtin_stack_top, which should return the real stack address. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 4, 2015 at 1:45 PM, Segher Boessenkool seg...@kernel.crashing.org wrote: On Tue, Aug 04, 2015 at 01:00:32PM -0700, H.J. Lu wrote: There is another issue with x86, maybe other targets. You can't get the real stack top when stack is realigned and -maccumulate-outgoing-args isn't used since ix86_expand_prologue will create and return another stack frame for __builtin_frame_address and __builtin_return_address. It will be wrong for __builtin_stack_top, which should return the real stack address. That's why I asked: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Yes, we want to use the argument pointer as shown in testcases included in my patch. -- H.J.
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 04, 2015 at 01:00:32PM -0700, H.J. Lu wrote: There is another issue with x86, maybe other targets. You can't get the real stack top when stack is realigned and -maccumulate-outgoing-args isn't used since ix86_expand_prologue will create and return another stack frame for __builtin_frame_address and __builtin_return_address. It will be wrong for __builtin_stack_top, which should return the real stack address. That's why I asked: You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. What would a C program do with this, that it cannot do with the frame address, that would be useful and cannot be much better done in straight assembler? Do you actually want to expose the argument pointer, maybe? Segher
Re: [PATCH] Add __builtin_stack_top
On Tue, Aug 04, 2015 at 11:50:00AM -0700, H.J. Lu wrote: The motivation of __builtin_stack_top is that frame_address requires a frame pointer register, which isn't desirable for x86. __builtin_stack_top doesn't require a frame pointer register. If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, you don't get crtl-accesses_prior_frames set either, and as far as I can see everything works fine? For __builtin_frame_address(0). You might have a reason why you want the entry stack address instead of the frame address, but you didn't really explain I think? Or I missed it. expand_builtin_return_addr sets crtl-accesses_prior_frames = 1; for __builtin_frame_address, which requires a frame pointer register. __builtin_stack_top doesn't set crtl-accesses_prior_frames and frame pointer register isn't required. Not if you have INITIAL_FRAME_ADDRESS_RTX. I don't see why the generic code cannot just use frame_pointer_rtx (instead of hard_frame_pointer_rtx) for a count of 0; but making it target-specific is certainly more conservative. You say i386 doesn't have that target macro defined currently. Yes I know; so change that? Or change the generic code, but that is much more testing. Segher
Re: [PATCH] Add __builtin_stack_top to x86 backend
On Thu, Jul 30, 2015 at 8:41 PM, H.J. Lu hongjiu...@intel.com wrote: On Tue, Jul 21, 2015 at 02:45:39PM -0700, H.J. Lu wrote: When __builtin_frame_address is used to retrieve the address of the function stack frame, the frame pointer is always kept, which wastes one register and 2 instructions. For x86-32, one less register means significant negative impact on performance. This patch adds a new builtin function, __builtin_ia32_stack_top, to x86 backend. It returns the stack address when the function is called. Any comments, feedbacks? Although this function is generic, but implementation is target specific. I submitted a generic patch: https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01859.html So far there are no interests from other backends. Here is a patch to implement __builtin_stack_top in x86 backend. We can update x86 backedn after it is added to middle-end. OK for trunk? I think that the discussion about generic implementation should come to some conclusion first. From the discussion, here was no resolution on which way to go. Uros.
