https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108448
Bug ID: 108448
Summary: GCC Elides Assignment to Pointer and memcpy
Product: gcc
Version: 11.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: gavin at yzena dot com
Target Milestone: ---
Host: x86_64
Target: x86_64
I have a stack allocator ("stackpool") that can automatically free all data
allocated after entry to a function. To do this, I pass in a pointer to the
function name to be the marker for the function start.
To debug, I also check the function name on exit.
This last item is segfaulting because the function name pointer is NULL. I ran
it through GDB, and the assignment to the allocated pointer ([1]) is skipped
entirely, but only when not compiling in plain Debug mode (CMake). I tried a
memcpy() ([2]), but it is *also* skipped.
I checked for undefined behavior with UBSan. It didn't catch any. I fully
acknowledge that there could be some, though.
To reproduce, run the following:
```
$ git clone https://git.yzena.com/Yzena/Yc.git
$ cd Yc
$ git config --local include.path ./.gitconfig
$ git submodule update --init --recursive
$ mkdir build
$ cd build
$ cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=<path/to/gcc>
-DYC_ENABLE_STACKTRACES=ON -DYC_ENABLE_LONG_TESTS=ON -DYC_BUILD_DOCS=OFF ..
$ make -j<ncores>
```
If the build fails, you have successfully reproduced the problem because there
is a program that uses that code used as part of the build (to generate a test
file). The program is `tests/strgen` (produced from `tests/strgen.c`), and the
command-line it runs is something like:
```
$ tests/container/../strgen tests/container/kjvbible.txt
tests/container/kjvbible.c kjvbible_array 1
```
To pre-emptively answer some questions: the function name pointer that is
passed in to `y_stackpool_enterFunc()` (the one with the elided assignment) is
not NULL, and neither is the stackpool.
But again, I wouldn't be surprised if there's some UB somewhere that is giving
GCC the "right" to elide this assignment. I just can't find where because I'm
not as expert in C as the GCC authors.
[1]:
https://git.yzena.com/Yzena/Yc/src/commit/6afdc86bd2c17f98b2f9e97e79e37fdf8c6b7708/src/alloc/stackpool.c#L441
[2]:
https://git.yzena.com/Yzena/Yc/src/commit/c9a855a0b6d9c5758e4d605977bdf571830132a2/src/alloc/stackpool.c#L442-L443
