Hi!
While expand_mult correctly checks for multiplication by zero,
expand_widening_mult does not and as floor_log2 (0) returns -1,
it attempts to expand a double-word shift by -1 and on ppc32
happens to return 2 w* 0 = 1. Usually earlier optimizations
just ensure that w* 0 isn't expanded using expand_widening_mult,
e.g. expand_mul_overflow which expands widening multiply separately
on highpart and lowpart of some value can end up with this.
I think it is better to fix this on the expand_widening_mult side
rather than having to bother with those special cases in every caller.
In 4.9, I think ubsan_expand_si_overflow_mul_check can trigger the
same thing.
Testcase for this is builtin-arith-overflow-14.c on ppc32.
Bootstrapped/regtested on x86_64-linux and i686-linux, tested on
the testcase with ppc cross. Ok for trunk/4.9?
2014-12-03 Jakub Jelinek <ja...@redhat.com>
PR c/59708
* expmed.c (expand_widening_mult): Return const0_rtx if
coeff is 0.
--- gcc/expmed.c.jj 2014-11-18 08:26:45.000000000 +0100
+++ gcc/expmed.c 2014-12-03 11:12:37.519297971 +0100
@@ -3289,6 +3289,9 @@ expand_widening_mult (machine_mode mode,
enum mult_variant variant;
struct algorithm algorithm;
+ if (coeff == 0)
+ return CONST0_RTX (mode);
+
/* Special case powers of two. */
if (EXACT_POWER_OF_2_OR_ZERO_P (coeff))
{
Jakub
