On Mon, Aug 10, 2020 at 10:36 AM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> To make amends for the recent (temporary) testsuite failure
> of my new gcc.target/i386/minmax-9.c when compiled with -m32,
> this patch improves the -m32 code we generate for the examples
> in that test case.
>
> Currently, smax(x,0) generates the very cool implementation:
>
> smax0: movl 4(%esp), %eax
> cltd
> notl %edx
> andl %edx, %eax
> ret
>
> But mysteriously, the related smin(x,0) uses a longer conditional
> move sequence:
>
> smin0: movl 4(%esp), %eax
> xorl %edx, %edx
> testl %eax, %eax
> setg %dl
> subl $1, %edx
> andl %edx, %eax
> ret
>
> With this patch, we now generate the much nicer:
>
> smin0: movl 4(%esp), %eax
> cltd
> andl %edx, %eax
> ret
>
> The trick is to expand smin(x,0) as "x < 0 ? x : 0" instead
> of the current "x <= 0 ? x : 0", as the former can take
> advantage of sign_bit_mask operations.
>
> The benefits can also be seen in QImode, where previously:
> smin0: movl 4(%esp), %edx
> testb %dl, %dl
> setg %al
> subl $1, %eax
> andl %edx, %eax
> ret
>
> now becomes:
>
> smin0: movl 4(%esp), %edx
> movl %edx, %eax
> sarb $7, %al
> andl %edx, %eax
> ret
>
>
> This patch has been tested on x86_64-pc-linux-gnu with a
> "make bootstrap" and "make -k check" with no new failures.
> Please let me know if I need to tweak the dejagnu directives;
> no other i386.exp test appears to explicitly specify -m32, but as
> a compile test, it's not unreasonable to check things on x86_64.
We don't use -m32 compile directives in the testsuite, please just use
"dg-do compile { target ia32 } }" dg directive.
BTW: Multilibs can be tested with:
make -j X check RUNTESTFLAGS="--target-board=unix\{,-m32}"
> OK for mainline?
OK with the above testcase fix.
Thanks,
Uros.
>
>
> 2020-08-10 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386-expand.c (ix86_expand_int_movcc): Expand
> signed MIN_EXPR against zero as "x < 0 ? x : 0" instead of
> "x <= 0 ? x : 0" to enable sign_bit_compare_p optimizations.
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/minmax-12.c: New test.
>
> Thanks in advance,
> Roger
> --
> Roger Sayle
> NextMove Software
> Cambridge, UK
>
