[Bug tree-optimization/100051] missed optimization for dead code elimination at -Os, -O2 and -O3 (vs. -O1)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100051

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to huadong from comment #6)
> There are still two questions:
> 1. With compilation option "-O2 -fno-strict-aliasing", which disables alias
> optimization, the code still has issues with its execution results.
> 2. Could you please help identify which specific line of code violates the
> aliasing rules? The GCC toolchain did not generate any compilation warnings
> regarding this.

You are storing an object of size uint32_t via reg_write to an object of
size 'char', the variable 'c'.  That invokes UB - this isn't about
strict aliasing, you are accessing the object 'c' out of bounds.

On Linux, 'c' isn't even aligned to 4 bytes, so you are storing to a random
location.