https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420
Bug ID: 106420
Summary: Missed optimization for comparisons
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: zero at smallinteger dot com
Target Milestone: ---
Created attachment 53339
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53339&action=edit
Sample code
When comparing different variables to the same constants, in some cases the
compiler could first combine the variables and then do a single compare. In
the sample given, two variables are compared against 7. In the slow path, GCC
produces the following with -O2.
cmp edi, 7
setg al
cmp esi, 7
setg dl
or eax, edx
movzx eax, al
ret
In the fast path, GCC produces this instead.
or edi, esi
xor eax, eax
cmp edi, 7
setg al
ret
Although the expression a > 7 || b > 7 is the same as (a | b) > 7, the latter
is better because it results in fewer instructions. A quick experiment shows
the latter also runs quite faster.
Verified with Godbolt for GCC trunk. Clang, ICC, and MSVC latest versions also
miss this opportunity as per Godbolt.
