https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102937
Bug ID: 102937
Summary: Miscompilation with -O3 and aliasing of char* and
size_t
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: drohr at jwdt dot org
Target Milestone: ---
Created attachment 51663
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51663&action=edit
testcase
The attached testcase miscompiles with GCC 11.2 when using -O3. It compiles
correctly with -O3, and also with GCC 10.3 using -O3. For reference the code is
also here: https://godbolt.org/z/cjr584sMo
The program prints a ptr twice, which should have been modified in between, but
this is not the case, thus it prints twice a nullptr:
FOO 0x(nil)
BAR 0x(nil)
By either commenting in the printf in line 51 (or also by outcommenting the
call to computePointerWithAlignment in line 50) the miscompilation disappears,
and the output is
FOO 0x(nil)
test 0x800
BAR 0x0x800
