Re: [RFC] Fix spill failure at -O on 64-bit Windows
Does this still happen after my patch from yesterday to use DF_LIVE in IRA? Yes, it even fails at -O2. Maybe add a cost-free dependency on the clobber, so that it's moved with the insn? Maybe. But I'm a little worried about (1) extending the lifetime of the hard register and (2) simply moving around a clobber of a hard register. And/or maybe punt on all likely_spilled hard registers if -fira-loop-pressure is not in effect, it's unlikely to be a win in any case. I'd like to keep the change minimal, in particular restricted to uninitialized hard registers. The failure mode is very specific (and might be unique to the 64-bit ABI on Windows) so I'm not sure it's worth generalizing. -- Eric Botcazou
Re: [RFC] Fix spill failure at -O on 64-bit Windows
On Wed, Oct 17, 2012 at 11:55 AM, Eric Botcazou wrote:
Maybe add a cost-free dependency on the clobber, so that it's moved
with the insn?
Maybe. But I'm a little worried about (1) extending the lifetime of the hard
register and (2) simply moving around a clobber of a hard register.
AFAIU the clobber is there because it was emitted specially for this
particular use of the hard register. So moving it around with the
reason why it's there in the first place would make sense to me.
I don't understand what you mean with extending the life of the hard
register in this case. If you move the clobber with the instruction,
the hard reg is dead before the clobber and after the insn that uses
it, just like when the insn is not hoisted from the loop. So you don't
extend the reg's live range if you move the clobber with it. And if
the register is not dead after the insn, it must be live through the
loop (otherwise it would be only partially available on the loop
entry, coming in from the loop latch) so you don't extend the live
range in that case either.
(One thing I don't fully understand here, though, is why the clobber
doesn't prevent the code motion to begin with. If you don't move the
clobber, that could change the semantics of the program, right? E.g if
you have before LIM something like this:
while (some_condition) { clobber(ecx); invariant_use (ecx); ... };
use (ecx);
and after LIM you have:
invariant_use (ecx);
while (some_condition) { clobber(ecx); ... };
use (ecx);
that's not the same program anymore if some_condition isn't true...
Not sure if that matters for this kind of uninitialized use.)
I'd like to keep the change minimal, in particular restricted to uninitialized
hard registers. The failure mode is very specific (and might be unique to the
64-bit ABI on Windows) so I'm not sure it's worth generalizing.
I've got a great dislike for special cases in the GCC code base, they
tend to grow into common practice at alarmingly fast rates :-)
But I suppose your point of view is more pragmatic in this peculiar case.
Ciao!
Steven
Re: [RFC] Fix spill failure at -O on 64-bit Windows
I don't understand what you mean with extending the life of the hard
register in this case. If you move the clobber with the instruction,
the hard reg is dead before the clobber and after the insn that uses
it, just like when the insn is not hoisted from the loop. So you don't
extend the reg's live range if you move the clobber with it. And if
the register is not dead after the insn, it must be live through the
loop (otherwise it would be only partially available on the loop
entry, coming in from the loop latch) so you don't extend the live
range in that case either.
In the former case, I agree that you don't really extend the live range.
But you nevertheless extend its span, i.e. you create a live range for the
hard register out of the loop, while it's supposed to be used only just before
the call in the argument preparation. In the latter case, I'm not sure that
your conclusion holds: why can't it be used in a second insn and die there?
(One thing I don't fully understand here, though, is why the clobber
doesn't prevent the code motion to begin with. If you don't move the
clobber, that could change the semantics of the program, right? E.g if
you have before LIM something like this:
while (some_condition) { clobber(ecx); invariant_use (ecx); ... };
use (ecx);
and after LIM you have:
invariant_use (ecx);
while (some_condition) { clobber(ecx); ... };
use (ecx);
that's not the same program anymore if some_condition isn't true...
Not sure if that matters for this kind of uninitialized use.)
I agree that moving an uninitialized use (of a hard register) without moving
the associated clobber is weird. But I think most RTL passes don't care and
this probably doesn't affect the correctness of the generated code.
I've got a great dislike for special cases in the GCC code base, they
tend to grow into common practice at alarmingly fast rates :-)
But I suppose your point of view is more pragmatic in this peculiar case.
I'm just realistic: you don't want to make substantial changes in a powerful
optimization pass just because of a corner case in an exotic language on a
semi-exotic platform. :-)
--
Eric Botcazou
[RFC] Fix spill failure at -O on 64-bit Windows
For the attached Ada testcase, the compiler aborts with a spill failure at -O:
p.adb: In function 'P.F':
p.adb:16:7: error: unable to find a register to spill in class 'DREG'
p.adb:16:7: error: this is the insn:
(insn 141 140 142 17 (parallel [
(set (reg:SI 0 ax [174])
(div:SI (subreg:SI (reg:DI 43 r14 [orig:81 iftmp.6 ] [81]) 0)
(reg:SI 41 r12 [orig:140 q__l.7 ] [140])))
(set (reg:SI 43 r14 [175])
(mod:SI (subreg:SI (reg:DI 43 r14 [orig:81 iftmp.6 ] [81]) 0)
(reg:SI 41 r12 [orig:140 q__l.7 ] [140])))
(clobber (reg:CC 17 flags))
]) p.adb:6 349 {*divmodsi4}
(expr_list:REG_DEAD (reg:SI 41 r12 [orig:140 q__l.7 ] [140])
(expr_list:REG_DEAD (reg:DI 43 r14 [orig:81 iftmp.6 ] [81])
(expr_list:REG_UNUSED (reg:SI 43 r14 [175])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))
+===GNAT BUG DETECTED==+
| 4.8.0 20121015 (experimental) [trunk revision 192447] (x86_64-pc-mingw32)
GCC error:|
| in spill_failure, at reload1.c:2124
The problem is that LIM hoists instructions preparing an argument register for
a call out of a loop, without moving the associated clobber:
(insn 249 248 250 33 (clobber (reg:SC 2 cx)) p.adb:12 -1
(nil))
[...]
(insn 253 251 254 33 (parallel [
(set (reg:DI 204)
(and:DI (reg:DI 2 cx)
(reg:DI 195)))
(clobber (reg:CC 17 flags))
]) p.adb:12 375 {*anddi_1}
(expr_list:REG_EQUAL (and:DI (reg:DI 2 cx)
(const_int -4294967296 [0x]))
(expr_list:REG_DEAD (reg:DI 2 cx)
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil)
Set in insn 253 is invariant (8), cost 8, depends on 6
Decided to move invariant 8 -- gain 8
This extends the lifetime of the hard register up to the beginning of the
function, causing reload to die on the complex division instruction.
The attached patch prevents the invariant from being hoisted in this very
particular case. Any better idea?
Tested on x86_64-suse-linux.
2012-10-15 Eric Botcazou ebotca...@adacore.com
* loop-invariant.c: Include target.h.
(check_dependency): Return false for an uninitialized argument register
that is likely to be spilled.
* Makefile.in (loop-invariant.o): Add $(TARGET_H).
2012-10-15 Eric Botcazou ebotca...@adacore.com
* gnat.dg/loop_optimization13.ad[sb]: New test.
* gnat.dg/loop_optimization13_pkg.ads: New helper.
--
Eric Botcazou-- { dg-do compile }
-- { dg-options -O }
with Loop_Optimization13_Pkg; use Loop_Optimization13_Pkg;
package body Loop_Optimization13 is
function F (A : Rec) return Rec is
N : constant Integer := A.V'Length / L;
Res : Rec
:= (True, new Complex_Vector' (0 .. A.V'Length / L - 1 = (0.0, 0.0)));
begin
for I in 0 .. L - 1 loop
for J in 0 .. N - 1 loop
Res.V (J) := Res.V (J) + A.V (I * N + J);
end loop;
end loop;
return Res;
end;
end Loop_Optimization13;with Ada.Numerics.Complex_Types; use Ada.Numerics.Complex_Types;
package Loop_Optimization13 is
type Complex_Vector is array (Integer range ) of Complex;
type Complex_Vector_Ptr is access Complex_Vector;
type Rec (Kind : Boolean := False) is record
case Kind is
when True = V : Complex_Vector_Ptr;
when False = null;
end case;
end record;
function F (A : Rec) return Rec;
end Loop_Optimization13;package Loop_Optimization13_Pkg is
L : Integer;
end Loop_Optimization13_Pkg;Index: Makefile.in
===
--- Makefile.in (revision 192447)
+++ Makefile.in (working copy)
@@ -3101,7 +3101,7 @@ loop-iv.o : loop-iv.c $(CONFIG_H) $(SYST
intl.h $(DIAGNOSTIC_CORE_H) $(DF_H) $(HASHTAB_H)
loop-invariant.o : loop-invariant.c $(CONFIG_H) $(SYSTEM_H) coretypes.h dumpfile.h \
$(RTL_H) $(BASIC_BLOCK_H) hard-reg-set.h $(CFGLOOP_H) $(EXPR_H) $(RECOG_H) \
- $(TM_H) $(TM_P_H) $(FUNCTION_H) $(FLAGS_H) $(DF_H) \
+ $(TM_H) $(TM_P_H) $(FUNCTION_H) $(FLAGS_H) $(DF_H) $(TARGET_H) \
$(OBSTACK_H) $(HASHTAB_H) $(EXCEPT_H) $(PARAMS_H) $(REGS_H) ira.h
cfgloopmanip.o : cfgloopmanip.c $(CONFIG_H) $(SYSTEM_H) $(RTL_H) \
$(BASIC_BLOCK_H) hard-reg-set.h $(CFGLOOP_H) \
Index: loop-invariant.c
===
--- loop-invariant.c (revision 192447)
+++ loop-invariant.c (working copy)
@@ -47,6 +47,7 @@ along with GCC; see the file COPYING3.
#include cfgloop.h
#include expr.h
#include recog.h
+#include target.h
#include function.h
#include flags.h
#include df.h
@@ -784,7 +785,22 @@ check_dependency (basic_block bb, df_ref
defs = DF_REF_CHAIN (use);
if (!defs)
-return true;
+{
+ unsigned int regno =
Re: [RFC] Fix spill failure at -O on 64-bit Windows
On Mon, Oct 15, 2012 at 7:45 PM, Eric Botcazou wrote: This extends the lifetime of the hard register up to the beginning of the function, causing reload to die on the complex division instruction. Does this still happen after my patch from yesterday to use DF_LIVE in IRA? The attached patch prevents the invariant from being hoisted in this very particular case. Any better idea? Maybe add a cost-free dependency on the clobber, so that it's moved with the insn? And/or maybe punt on all likely_spilled hard registers if -fira-loop-pressure is not in effect, it's unlikely to be a win in any case. Ciao! Steven
