On Tue, 6 Oct 2020 at 18:02, Richard Earnshaw
<richard.earns...@foss.arm.com> wrote:
>
> On 29/09/2020 20:50, Christophe Lyon via Gcc-patches wrote:
> > When mi_delta is > 255 and -mpure-code is used, we cannot load delta
> > from code memory (like we do without -mpure-code).
> >
> > This patch builds the value of mi_delta into r3 with a series of
> > movs/adds/lsls.
> >
> > We also do some cleanup by not emitting the function address and delta
> > via .word directives at the end of the thunk since we don't use them
> > with -mpure-code.
> >
> > No need for new testcases, this bug was already identified by
> > eg. pr46287-3.C
> >
> > 2020-09-29 Christophe Lyon <christophe.l...@linaro.org>
> >
> > gcc/
> > * config/arm/arm.c (arm_thumb1_mi_thunk): Build mi_delta in r3 and
> > do not emit function address and delta when -mpure-code is used.
>
Hi Richard,
Thanks for your comments.
> There are some optimizations you can make to this code.
>
> Firstly, for values between 256 and 510 (inclusive), it would be better
> to just expand a mov of 255 followed by an add.
I now see the splitted for the "Pe" constraint which I hadn't noticed
before, so I can write something similar indeed.
However, I'm note quite sure to understand the benefit in the split
when -mpure-code is NOT used.
Consider:
int f3_1 (void) { return 510; }
int f3_2 (void) { return 511; }
Compile with -O2 -mcpu=cortex-m0:
f3_1:
movs r0, #255
lsls r0, r0, #1
bx lr
f3_2:
ldr r0, .L4
bx lr
The splitter makes the code bigger, does it "compensate" for this by
not having to load the constant?
Actually the constant uses 4 more bytes, which should be taken into
account when comparing code size,
so f3_1 uses 6 bytes, and f3_2 uses 8, so as you say below three
thumb1 instructions would be equivalent in size compared to loading
from the literal pool. Should the 256-510 range be extended?
> This is also true for
> the literal pools alternative as well, so should be handled before all
> this.
I am not sure what you mean: with -mpure-code, the above sample is compiled as:
f3_1:
movs r0, #255
lsls r0, r0, #1
bx lr
f3_2:
movs r0, #1
lsls r0, r0, #8
adds r0, r0, #255
bx lr
so the "return 510" case is already handled as without -mpure-code.
> I also suspect (but haven't check) that the base adjustment will
> most commonly be a multiple of the machine word size (ie 4). If that is
> the case then you could generate n/4 and then shift it left by 2 for an
> even greater range of literals.
I can see there is provision for this in the !TARGET_THUMB1_ONLY case,
I'll update my patch.
> More generally, any sequence of up to
> three thumb1 instructions will be no larger, and probably as fast as the
> existing literal pool fall back.
>
> Secondly, if the value is, for example, 65536 (0x10000), your code will
> emit a mov followed by two shift-by-8 instructions; the two shifts could
> be merged into a single shift-by-16.
Right, I'll try to make use of thumb_shiftable_const.
> Finally, I'd really like to see some executable tests for this, if at
> all possible.
I mentioned pr46287-3.C, but that's not the only existing testcase
that showed the problem. There are also:
g++.dg/opt/thunk1.C
g++.dg/ipa/pr46984.C
g++.dg/torture/pr46287.C
g++.dg/torture/pr45699.C
Do you want that I copy one of these in the arm subdir and add
-mpure-code in dg-options?
Thanks,
Christophe
> R.
>
> >
> > k# (use "git pull" to merge the remote branch into yours)
> > ---
> > gcc/config/arm/arm.c | 91
> > +++++++++++++++++++++++++++++++++++++---------------
> > 1 file changed, 66 insertions(+), 25 deletions(-)
> >
> > diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> > index ceeb91f..62abeb5 100644
> > --- a/gcc/config/arm/arm.c
> > +++ b/gcc/config/arm/arm.c
> > @@ -28342,9 +28342,43 @@ arm_thumb1_mi_thunk (FILE *file, tree,
> > HOST_WIDE_INT delta,
> > {
> > if (mi_delta > 255)
> > {
> > - fputs ("\tldr\tr3, ", file);
> > - assemble_name (file, label);
> > - fputs ("+4\n", file);
> > + /* With -mpure-code, we cannot load delta from the constant
> > + pool: we build it explicitly. */
> > + if (target_pure_code)
> > + {
> > + bool mov_done_p = false;
> > + int i;
> > +
> > + /* Emit upper 3 bytes if needed. */
> > + for (i = 0; i < 3; i++)
> > + {
> > + int byte = (mi_delta >> (8 * (3 - i))) & 0xff;
> > +
> > + if (byte)
> > + {
> > + if (mov_done_p)
> > + asm_fprintf (file, "\tadds\tr3, #%d\n", byte);
> > + else
> > + asm_fprintf (file, "\tmovs\tr3, #%d\n", byte);
> > + mov_done_p = true;
> > + }
> > +
> > + if (mov_done_p)
> > + asm_fprintf (file, "\tlsls\tr3, #8\n");
> > + }
> > +
> > + /* Emit lower byte if needed. */
> > + if (!mov_done_p)
> > + asm_fprintf (file, "\tmovs\tr3, #%d\n", mi_delta & 0xff);
> > + else if (mi_delta & 0xff)
> > + asm_fprintf (file, "\tadds\tr3, #%d\n", mi_delta & 0xff);
> > + }
> > + else
> > + {
> > + fputs ("\tldr\tr3, ", file);
> > + assemble_name (file, label);
> > + fputs ("+4\n", file);
> > + }
> > asm_fprintf (file, "\t%ss\t%r, %r, r3\n",
> > mi_op, this_regno, this_regno);
> > }
> > @@ -28380,30 +28414,37 @@ arm_thumb1_mi_thunk (FILE *file, tree,
> > HOST_WIDE_INT delta,
> > fputs ("\tpop\t{r3}\n", file);
> >
> > fprintf (file, "\tbx\tr12\n");
> > - ASM_OUTPUT_ALIGN (file, 2);
> > - assemble_name (file, label);
> > - fputs (":\n", file);
> > - if (flag_pic)
> > +
> > + /* With -mpure-code, we don't need to emit literals for the
> > + function address and delta since we emitted code to build
> > + them. */
> > + if (!target_pure_code)
> > {
> > - /* Output ".word .LTHUNKn-[3,7]-.LTHUNKPCn". */
> > - rtx tem = XEXP (DECL_RTL (function), 0);
> > - /* For TARGET_THUMB1_ONLY the thunk is in Thumb mode, so the PC
> > - pipeline offset is four rather than eight. Adjust the offset
> > - accordingly. */
> > - tem = plus_constant (GET_MODE (tem), tem,
> > - TARGET_THUMB1_ONLY ? -3 : -7);
> > - tem = gen_rtx_MINUS (GET_MODE (tem),
> > - tem,
> > - gen_rtx_SYMBOL_REF (Pmode,
> > - ggc_strdup (labelpc)));
> > - assemble_integer (tem, 4, BITS_PER_WORD, 1);
> > - }
> > - else
> > - /* Output ".word .LTHUNKn". */
> > - assemble_integer (XEXP (DECL_RTL (function), 0), 4, BITS_PER_WORD, 1);
> > + ASM_OUTPUT_ALIGN (file, 2);
> > + assemble_name (file, label);
> > + fputs (":\n", file);
> > + if (flag_pic)
> > + {
> > + /* Output ".word .LTHUNKn-[3,7]-.LTHUNKPCn". */
> > + rtx tem = XEXP (DECL_RTL (function), 0);
> > + /* For TARGET_THUMB1_ONLY the thunk is in Thumb mode, so the PC
> > + pipeline offset is four rather than eight. Adjust the offset
> > + accordingly. */
> > + tem = plus_constant (GET_MODE (tem), tem,
> > + TARGET_THUMB1_ONLY ? -3 : -7);
> > + tem = gen_rtx_MINUS (GET_MODE (tem),
> > + tem,
> > + gen_rtx_SYMBOL_REF (Pmode,
> > + ggc_strdup (labelpc)));
> > + assemble_integer (tem, 4, BITS_PER_WORD, 1);
> > + }
> > + else
> > + /* Output ".word .LTHUNKn". */
> > + assemble_integer (XEXP (DECL_RTL (function), 0), 4,
> > BITS_PER_WORD, 1);
> >
> > - if (TARGET_THUMB1_ONLY && mi_delta > 255)
> > - assemble_integer (GEN_INT(mi_delta), 4, BITS_PER_WORD, 1);
> > + if (TARGET_THUMB1_ONLY && mi_delta > 255)
> > + assemble_integer (GEN_INT(mi_delta), 4, BITS_PER_WORD, 1);
> > + }
> > }
> > else
> > {
> >
>
