From: Thomas Schwinge <tschwinge@baylibre.com>
To: Richard Sandiford <richard.sandiford@arm.com>
Cc: jlaw@ventanamicro.com, rdapp.gcc@gmail.com,
gcc-patches@gcc.gnu.org, Tom de Vries <tdevries@suse.de>,
Roger Sayle <roger@nextmovesoftware.com>
Subject: Re: nvptx vs. [PATCH] Add a late-combine pass [PR106594]
Date: Thu, 27 Jun 2024 22:27:21 +0200 [thread overview]
Message-ID: <87r0ci2kt2.fsf@euler.schwinge.ddns.net> (raw)
In-Reply-To: <87r0citjoy.fsf@euler.schwinge.ddns.net>
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Hi!
On 2024-06-27T18:49:17+0200, I wrote:
> On 2023-10-24T19:49:10+0100, Richard Sandiford <richard.sandiford@arm.com> wrote:
>> This patch adds a combine pass that runs late in the pipeline.
[After sending, I realized I replied to a previous thread of this work.]
> I've beek looking a bit through recent nvptx target code generation
> changes for GCC target libraries, and thought I'd also share here my
> findings for the "late-combine" changes in isolation, for nvptx target.
>
> First the unexpected thing:
So much for "unexpected thing" -- next level of unexpected here...
Appreciated if anyone feels like helping me find my way through this, but
I totally understand if you've got other things to do.
> there are a few cases where we now see unused
> registers get declared, for example (random) in
> 'nvptx-none/newlib/libc/libm_a-s_modf.o:modf'
I first looked into a simpler case: newlib 'libc/locale/lnumeric.c'.
Here we get the following 'diff' for '*.s' for
'-fno-late-combine-instructions' vs. (default)
'-flate-combine-instructions':
.visible .func (.param.u32 %value_out) __numeric_load_locale (.param.u64 %in_ar0, .param.u64 %in_ar1, .param.u64 %in_ar2, .param.u64 %in_ar3)
{
.reg.u32 %value;
.reg.u64 %ar0;
ld.param.u64 %ar0, [%in_ar0];
.reg.u64 %ar1;
ld.param.u64 %ar1, [%in_ar1];
.reg.u64 %ar2;
ld.param.u64 %ar2, [%in_ar2];
.reg.u64 %ar3;
ld.param.u64 %ar3, [%in_ar3];
+ .reg.u32 %r22;
.file 2 "../../../source-gcc/newlib/libc/locale/lnumeric.c"
.loc 2 89 1
mov.u32 %value, 0;
st.param.u32 [%value_out], %value;
ret;
}
Clearly, '%r22' is unused. However, looking at the source code (manually
trimmed):
int
__numeric_load_locale (struct __locale_t *locale, const char *name ,
void *f_wctomb, const char *charset)
{
int ret;
struct lc_numeric_T nm;
char *bufp = NULL;
#ifdef __CYGWIN__
[...]
#else
/* TODO */
#endif
return ret;
}
..., and adding '-Wall' (why isn't top-level/newlib build system doing
that...):
[...]
../../../source-gcc/newlib/libc/locale/lnumeric.c:88:10: warning: ‘ret’ is used uninitialized [-Wuninitialized]
88 | return ret;
| ^~~
../../../source-gcc/newlib/libc/locale/lnumeric.c:48:7: note: ‘ret’ was declared here
48 | int ret;
| ^~~
Uh. Given nothing else is going on in that function, I suppose '%r22'
relates to the uninitialized 'ret' -- and given undefined behavior, GCC
of course is fine to emit an unused 'reg' in that case...
But: should we expect '-fno-late-combine-instructions' vs.
'-flate-combine-instructions' to behave in the same way? (After all,
'%r22' remains unused also with '-flate-combine-instructions', and
doesn't need to be emitted.) This could, of course, also be a nvptx back
end issue?
I'm happy to supply any dump files etc. Also, 'tmp-libc_a-lnumeric.i.xz'
is attached if you'd like to reproduce this with your own nvptx target
'cc1':
$ [...]/configure --target=nvptx-none --enable-languages=c
$ make -j12 all-gcc
$ gcc/cc1 -fpreprocessed tmp-libc_a-lnumeric.i -quiet -dumpbase tmp-libc_a-lnumeric.c -dumpbase-ext .c -misa=sm_30 -g -O2 -fno-builtin -o tmp-libc_a-lnumeric.s -fdump-rtl-all # -fno-late-combine-instructions
Grüße
Thomas
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next prev parent reply other threads:[~2024-06-27 20:27 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-24 18:49 Richard Sandiford
2023-11-30 14:10 ` Ping: " Richard Sandiford
2023-12-11 15:23 ` Richard Sandiford
2023-12-11 16:18 ` Robin Dapp
2023-12-30 15:35 ` Ping^3: " Richard Sandiford
2024-01-01 3:11 ` YunQiang Su
2024-01-05 10:10 ` YunQiang Su
2023-12-30 18:13 ` Segher Boessenkool
2024-01-02 9:47 ` Richard Sandiford
2024-06-24 19:37 ` Segher Boessenkool
2024-06-25 10:31 ` Richard Biener
2024-06-25 17:22 ` YunQiang Su
2024-01-03 4:20 ` Jeff Law
2024-01-05 17:35 ` Richard Sandiford
2024-01-08 5:03 ` Jeff Law
2024-01-08 11:52 ` Richard Sandiford
2024-01-08 16:14 ` Jeff Law
2024-01-08 16:59 ` Richard Sandiford
2024-01-08 17:10 ` Jeff Law
2024-01-08 19:11 ` Richard Sandiford
2024-01-08 21:42 ` Jeff Law
2024-01-10 13:01 ` Richard Sandiford
2024-01-10 13:35 ` Richard Biener
2024-01-10 16:27 ` Jeff Law
2024-01-10 16:40 ` Jeff Law
2024-06-21 4:50 ` Hongtao Liu
2024-06-27 16:49 ` nvptx vs. " Thomas Schwinge
2024-06-27 20:27 ` Thomas Schwinge [this message]
2024-06-27 21:20 ` Thomas Schwinge
2024-06-27 22:41 ` Thomas Schwinge
2024-06-28 14:01 ` Richard Sandiford
2024-06-28 16:48 ` Richard Sandiford
2024-07-01 11:55 ` Thomas Schwinge
2024-07-01 11:55 ` WIP Move 'pass_fast_rtl_dce' from 'pass_postreload' into 'pass_late_compilation' (was: nvptx vs. [PATCH] Add a late-combine pass [PR106594]) Thomas Schwinge
2024-06-28 6:07 ` nvptx vs. [PATCH] Add a late-combine pass [PR106594] Roger Sayle
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