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Tweak tree-ssa-math-opts.c to solve PR target/102117.

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This patch resolves PR target/102117 on s390.  The problem is that
some of the functionality of GCC's RTL expanders is no longer triggered
following the transition to tree SSA form.  On s390, unsigned widening
multiplications are converted into WIDEN_MULT_EXPR (aka w* in tree dumps),
but signed widening multiplies are left in their original form, which
alas doesn't benefit from the clever logic in expand_widening_mult.

The fix is to teach convert_mult_to_widen, that RTL expansion can
synthesize a signed widening multiplication if the target provides
a suitable umul_widen_optab.

On s390-linux-gnu with -O2 -m64, the code in the bugzilla PR currently
generates:

imul128:
        stmg    %r12,%r13,96(%r15)
        srag    %r0,%r4,63
        srag    %r1,%r3,63
        lgr     %r13,%r3
        mlgr    %r12,%r4
        msgr    %r1,%r4
        msgr    %r0,%r3
        lgr     %r4,%r12
        agr     %r1,%r0
        lgr     %r5,%r13
        agr     %r4,%r1
        stmg    %r4,%r5,0(%r2)
        lmg     %r12,%r13,96(%r15)
        br      %r14

but with this patch should now generate the more efficient:

imul128:
        lgr     %r1,%r3
        mlgr    %r0,%r4
        srag    %r5,%r3,63
        ngr     %r5,%r4
        srag    %r4,%r4,63
        sgr     %r0,%r5
        ngr     %r4,%r3
        sgr     %r0,%r4
        stmg    %r0,%r1,0(%r2)
        br      %r14

2021-11-21  Roger Sayle  <roger@nextmovesoftware.com>
	    Robin Dapp  <rdapp@linux.ibm.com>

gcc/ChangeLog
	PR target/102117
	* tree-ssa-math-opts.c (convert_mult_to_widen): Recognize
	signed WIDEN_MULT_EXPR if the target supports umul_widen_optab.

gcc/testsuite/ChangeLog
	PR target/102117
	* gcc.target/s390/mul-wide.c: New test case.
	* gcc.target/s390/umul-wide.c: New test case.
This commit is contained in:
Roger Sayle 2021-11-21 11:40:08 +00:00
parent f658f1d7a2
commit dc915b361b
3 changed files with 28 additions and 1 deletions
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9
gcc/testsuite/gcc.target/s390/mul-wide.c Normal file
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@ -0,0 +1,9 @@
/* { dg-do compile } */
/* { dg-options "-O2 -m64 -fdump-tree-optimized" } */
__int128 foo(long long a, long long b)
{
return (__int128)a * (__int128)b;
}
/* { dg-final { scan-tree-dump " w\\* " "optimized" } } */

9
gcc/testsuite/gcc.target/s390/umul-wide.c Normal file
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@ -0,0 +1,9 @@
/* { dg-do compile } */
/* { dg-options "-O2 -m64 -fdump-tree-optimized" } */
unsigned __int128 foo(unsigned long long a, unsigned long long b)
{
return (unsigned __int128)a * (unsigned __int128)b;
}
/* { dg-final { scan-tree-dump " w\\* " "optimized" } } */

11
gcc/tree-ssa-math-opts.c
View File

@ -2723,7 +2723,16 @@ convert_mult_to_widen (gimple *stmt, gimple_stmt_iterator *gsi)
from_unsigned1 = from_unsigned2 = false;
}
else
return false;
{
/* Expand can synthesize smul_widen_optab if the target
supports umul_widen_optab. */
op = umul_widen_optab;
handler = find_widening_optab_handler_and_mode (op, to_mode,
from_mode,
&actual_mode);
if (handler == CODE_FOR_nothing)
return false;
}
}
/* Ensure that the inputs to the handler are in the correct precison