https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95097
Bug ID: 95097
Summary: Missed optimization with bitfield value ranges
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: bugdal at aerifal dot cx
Target Milestone: ---
#include <stdint.h>
struct foo {
uint32_t x:20;
};
int bar(struct foo f)
{
if (f.x) {
uint32_t y = (uint32_t)f.x*4096;
if (y<200) return 1;
else return 2;
}
return 3;
}
Here, truth of the condition f.x implies y>=4096, but GCC does not DCE the
y<200 test and return 1 codepath.
I actually had this come up in real world code, where I was considering use of
an inline function with nontrivial low size cases when a "page count" bitfield
is zero, where I expected these nontrivial cases to be optimized out based on
already having tested that the page count being nonzero, but GCC was unable to
do it. LLVM/clang does it.
