https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97434
Bug ID: 97434
Summary: Missed dead code optimization from data flow analysis
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jwerner at chromium dot org
Target Milestone: ---
I found a pretty simple case where GCC cannot optimize out a redundant check.
I've reduced it to the following minimal test case:
unsigned int random_number(void);
void eliminate_me(void);
void main(void)
{
unsigned int a = random_number();
unsigned int b = random_number();
if (b > a)
return;
int x = b - 8;
if (x > 0 && x > a)
eliminate_me();
}
I think it should be really easy to prove that eliminate_me() cannot be called,
because x can never be greater than a (otherwise b would have also been greater
than a and the function would have terminated earlier). I don't know anything
about how compilers do data flow analysis in detail, but GCC can usually figure
out so much that I'm surprised it cannot figure out this one.
