https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91789
Bug ID: 91789
Summary: Value ranges determined from comparisons not used
transitively
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
Take the following code:
int foo(int a, int b)
{
if (b < a)
__builtin_unreachable();
if (a < 0)
return -1;
if (b < 0)
return 0;
return 1;
}
The compiler should be able to determine that the b < 0 can never be true. At
that point in the code a >= 0 and b >= a, therefore transitively b >= 0.
The problem is not tied to __builtin_unreachable as can be seen by changing the
code slightly:
int foo(int a, int b)
{
if (b < a)
return 2;
if (a < 0)
return -1;
if (b < 0)
return 0;
return 1;
}
After the initial test b < a is handled there is still a threeway comparison.
The problem can be seen with 9.2.1 as well as the current trunk version. clang
8.0.0 generates pretty much the same code as gcc.
