https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99954
Bug ID: 99954
Summary: Copy loop over array of unions at -O3 generates memcpy
instead of memmove, resulting in incorrect code
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: doublef.mobile at gmail dot com
Target Milestone: ---
Created attachment 50521
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50521&action=edit
test case (should exit successfully but halts on assertion error)
The attached code is intended to generate an array of e.g. 1,1,2,3,4,5... (if
called without arguments) but instead generates e.g. 1,1,1,1,1 as the loop in
move() is incorrectly optimized to memcpy. The issue has been detected on x86,
amd64 and mips.
As checked on godbolt.org, issue applies to GCC 7+ (6 and below correctly
generate memmove); using -O2 "fixes" the issue on GCC 9 and below but not on
10.
If "#define CONTAINER_KIND struct" is used instead, the issue is not triggered.
