https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114700
Bug ID: 114700
Summary: Front-end optimization generates wrong code with
-ftrapv.
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: lin1.hu at intel dot com
Target Milestone: ---
We test GCC vs Clang with -ftrapv, and test is
z = c - y - c + a + y - b;
https://godbolt.org/z/EW1xTsazG
We think the clang is right, the overflow judgment should be performed after
each operation. But the front-end generates a - b directly, looks like there's
a bug in the front-end's handling of -ftrapv.
-ftrapv
This option generates traps for signed overflow on addition, subtraction,
multiplication operations. The options -ftrapv and -fwrapv override each other,
so using -ftrapv -fwrapv on the command-line results in -fwrapv being
effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.
We have another question, we found the front-end won't optimize z = c - y + a -
c, while z = c - y - c + a will, is this for any particular reason or is it a
bug?
