[gcc r12-5436] Tweak tree-ssa-math-opts.c to solve PR target/102117.

[Roger Sayle <sayle@gcc.gnu.org>]

https://gcc.gnu.org/g:dc915b361bbc99da83fc53db7f7e0e28d0ce12c8

commit r12-5436-gdc915b361bbc99da83fc53db7f7e0e28d0ce12c8
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sun Nov 21 11:40:08 2021 +0000

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

Diff:
---
 gcc/testsuite/gcc.target/s390/mul-wide.c  |  9 +++++++++
 gcc/testsuite/gcc.target/s390/umul-wide.c |  9 +++++++++
 gcc/tree-ssa-math-opts.c                  | 11 ++++++++++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/gcc.target/s390/mul-wide.c b/gcc/testsuite/gcc.target/s390/mul-wide.c
new file mode 100644
index 00000000000..8a2092e01c4
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/mul-wide.c
@@ -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" } } */
diff --git a/gcc/testsuite/gcc.target/s390/umul-wide.c b/gcc/testsuite/gcc.target/s390/umul-wide.c
new file mode 100644
index 00000000000..33a74e52170
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/umul-wide.c
@@ -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" } } */
diff --git a/gcc/tree-ssa-math-opts.c b/gcc/tree-ssa-math-opts.c
index cc8496c3c32..6131824191f 100644
--- a/gcc/tree-ssa-math-opts.c
+++ b/gcc/tree-ssa-math-opts.c
@@ -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