https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107107
Bug ID: 107107
Summary: Wrong codegen from TBAA when stores to distinct
same-mode types are collapsed?
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: bugdal at aerifal dot cx
Target Milestone: ---
Created attachment 53646
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53646&action=edit
original test case by supercat
The attached test case is from user supercat on Stack Overflow (original
source:
https://stackoverflow.com/questions/42178179/will-casting-around-sockaddr-storage-and-sockaddr-in-break-strict-aliasing/42178347?noredirect=1#comment130509588_42178347,
https://godbolt.org/z/83v4ssrn4) and demonstrates wrong TBAA apparently
assuming an object of type long long was not modified after the code path
modifying it was collapsed with a different code path performing the
modification via an lvalue of type long.
On 64-bit targets, the test program outputs 1/2 with optimization levels that
enable -fstrict-aliasing. The expected output is 2/2. Using
-fno-strict-aliasing fixes it.
I have not checked this myself, but according to others who have looked at the
test case, the regression came between GCC 4.7 and 4.8.
