https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68294
Bug ID: 68294
Summary: gcc cannot deduce (a | b) != 0 from (a != 0 && b != 0)
Product: gcc
Version: 5.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: fuz at fuz dot su
Target Milestone: ---
I implemented Stein's binary gcd algorithm, which contains code like this:
int u, v, k;
/* ... */
if (u == 0 || v == 0)
return (u | v);
k = ffs(u | v) - 1;
Sadly, for the ffs() call, gcc emits (on x86 where behaviour of bsf is unclear
if the operand is zero) code for the case when u | v is zero, even though this
possibility has been ruled out in the if-statement before. Adding a redundant
clause
if ((u | v) == 0 || u == 0 || v == 0)
to the if-statement makes the compiler omit the extra code it emits above, but
then it emits an extra unneeded test for (u | v) == 0.
It would be great if gcc would catch this.
