https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115924
Bug ID: 115924
Summary: Failure to optimize back and forth shifts of several
operands by the same amount with operations in the
middle to ands
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
#include <stdint.h>
int32_t f(int32_t i2, int32_t aa)
{
return ((i2 >> 17) + (aa >> 17)) << 17;
}
int32_t f2(int32_t i2, int32_t aa)
{
return ((i2 & -(1 << 17)) + aa) & -(1 << 17);
}
bool f3(int32_t i2, int32_t aa)
{
return f(i2, aa) == f2(i2, aa);
}
f can be optimized to f2, and consequently f3 to `return true;`. This
transformation is done by LLVM, but not by GCC.
(note: this seems logically generalizable to amounts other than 17, and pretty
much any operation after right shifting the two operands before shifting them
back (e.g. replacing `+` with `-`, `^` or many other more complex operations))
