[PATCH] tree-ssa-math-opts: Pattern recognize hand written __builtin_mul_overflow_p with same unsigned types even when target just has highpart umul [PR101856]

[Jakub Jelinek via Gcc-patches]

Hi!

As can be seen on the following testcase, we pattern recognize it on
i?86/x86_64 as return __builtin_mul_overflow_p (x, y, 0UL) and avoid
that way the extra division, but don't do it e.g. on aarch64 or ppc64le,
even when return __builtin_mul_overflow_p (x, y, 0UL); actually produces
there better code.  The reason for testing the presence of the optab
handler is to make sure the generated code for it is short to ensure
we don't actually pessimize code instead of optimizing it.
But, we have one case that the internal-fn.cc .MUL_OVERFLOW expansion
handles nicely, and that is when arguments/result is the same mode
TYPE_UNSIGNED type, we only use IMAGPART_EXPR of it (i.e.
__builtin_mul_overflow_p rather than __builtin_mul_overflow) and
umul_highpart_optab supports the particular mode, in that case
we emit comparison of the highpart umul result against zero.

So, the following patch matches what we do in internal-fn.cc and
also pattern matches __builtin_mul_overflow_p if
1) we only need the flag whether it overflowed (i.e. !use_seen)
2) it is unsigned (i.e. !cast_stmt)
3) umul_highpart is supported for the mode

Bootstrapped/regtested on x86_64-linux, i686-linux, aarch64-linux and
powerpc64le-linux, ok for trunk?

2023-05-19  Jakub Jelinek  <ja...@redhat.com>

        PR tree-optimization/101856
        * tree-ssa-math-opts.cc (match_arith_overflow): Pattern detect
        unsigned __builtin_mul_overflow_p even when umulv4_optab doesn't
        support it but umul_highpart_optab does.

        * gcc.dg/tree-ssa/pr101856.c: New test.

--- gcc/tree-ssa-math-opts.cc.jj        2023-05-17 20:57:59.537914382 +0200
+++ gcc/tree-ssa-math-opts.cc   2023-05-18 12:04:09.332336899 +0200
@@ -4074,7 +4074,10 @@ match_arith_overflow (gimple_stmt_iterat
                            TYPE_MODE (type)) == CODE_FOR_nothing)
       || (code == MULT_EXPR
          && optab_handler (cast_stmt ? mulv4_optab : umulv4_optab,
-                           TYPE_MODE (type)) == CODE_FOR_nothing))
+                           TYPE_MODE (type)) == CODE_FOR_nothing
+         && (use_seen
+             || cast_stmt
+             || !can_mult_highpart_p (TYPE_MODE (type), true))))
     {
       if (code != PLUS_EXPR)
        return false;
--- gcc/testsuite/gcc.dg/tree-ssa/pr101856.c.jj 2023-05-18 11:57:17.681206745 
+0200
+++ gcc/testsuite/gcc.dg/tree-ssa/pr101856.c    2023-05-18 11:56:51.662577752 
+0200
@@ -0,0 +1,11 @@
+/* PR tree-optimization/101856 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump " .MUL_OVERFLOW " "optimized" { target i?86-*-* 
x86_64-*-* aarch64*-*-* powerpc64le-*-* } } } */
+
+int
+foo (unsigned long x, unsigned long y)
+{
+  unsigned long z = x * y;
+  return z / y != x;
+}

        Jakub