https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107115
Bug ID: 107115
Summary: Wrong codegen from TBAA under stores that change
effective type?
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 53648
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53648&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#comment130510083_42178347,
https://godbolt.org/z/jfv1Ge6v4) and demonstrates what appears to be wrong TBAA
optimization on an object with allocated storage whose effective type changes
under stores. It was first presented as another example of this kind of problem
alongside the example that became
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107107, but it seems likely that
the root cause is distinct.
Reportedly clang/LLVM also transforms this example wrong.
On 64-bit targets, the test program outputs 2/1 with optimization levels that
enable -fstrict-aliasing. The expected output is 2/2. Using
-fno-strict-aliasing fixes it.
