per-function-optimize-attribute-abi-mismatch

__attribute__((optimize("O0"))) silently breaks the ABI on GCC

When to use

You're trying to isolate a miscompile in a single helper function by forcing it to lower optimization, using one of these forms inside a translation unit that is otherwise built at -O2 or higher:

__attribute__((optimize("O0")))
static void Helper(uint8_t* Dst, const uint8_t* Src, int N) { ... }

or equivalently:

#pragma GCC push_options
#pragma GCC optimize("O0")
static void Helper(...) { ... }
#pragma GCC pop_options

and the program now crashes — usually SIGSEGV — on the first call into Helper, before any of its body runs, even though the same code worked at -O2.

Problem

GCC's per-function optimize attribute changes the optimization level of the callee, but it does not re-align the ABI between that callee and its -O2 caller. Concretely:

• -O0 GCC emits a full stack frame with a frame pointer, spills all arguments to memory, and does not use the x86-64 SysV red zone.
• -O2 GCC elides the frame pointer, keeps arguments in registers, and may use the red zone.
• The caller, built at -O2, passes arguments and return address under -O2 assumptions. The callee reads them under -O0 assumptions. The mismatch shows up as reading 0 or garbage from a pointer argument, or a segfault on return (frame pointer mismatch).

Crucially, there is no compiler warning. The attribute silently produces a working-looking binary that crashes at runtime.

On Clang, __attribute__((optnone)) exists and is better-behaved about this, but it's also more restrictive (disables all opts, not just some) and is not a fix-by-dropping-level tool.

Solution

If you really need one function at a lower optimization level, pick one of these, in order of preference:

1. Don't. If you reached for optimize("O0") to diagnose a miscompile, see stop-chasing-the-optimizer-reduce-instead — reduce the repro and fix the real bug (usually UB) instead.
2. Isolate the function in its own TU, compiled at -O1. Move Helper into helper.cpp and add a per-file flag in the build system: