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Fwiw clang has a `clang::optnone` attribute to disable all optimizations on a per-function basis, and GCC has the fantastic `gnu::optimize` attribute which allo

Fwiw clang has a `clang::optnone` attribute to disable all optimizations on a per-function basis, and GCC has the fantastic `gnu::optimize` attribute which allows you to add or remove optimizations by name, or set the optimization level regardless of compiler flags. `gnu::optimize(0)` is similar to that clang flag. Clang also has `clang::no_builtins` to disable specifically the memcpy and memset optimizations.

leni536 2y ago
"The optimize attribute should be used for debugging purposes only. It is not suitable in production code. " https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attribute...
- Conscat 2y ago
  That's an interesting note. I wonder why they claim this. As far as I know, `[[gnu::optimize("-fno-tree-loop-distribute-patterns")]]` (or the equivalent #pragma) is required for implementing a memcpy function in C unless you do something funky with the build system.
  - leni536 2y ago
    Maybe that's applied to the TU that defines it? I don't see it in the glibc sources.
    - anarazel 2y ago
      It's indeed used: Definition of macro: https://sourceware.org/git/?p=glibc.git;a=blob;f=include/libc-symbols.h;h=4367aa6740bff6fc6b42b562f5d9a95b6690bc50;hb=HEAD#l808 https://sourceware.org/git/?p=glibc.git;a=blob;f=include/lib... Use: https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memmove.c;h=3fad5f11acdf5d8a5ae1436c1e577f050c5d5193;hb=HEAD#l42 https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memm... For a bunch of other places -fno-builtin-* seems to be used.
      - leni536 2y ago
        My guess is that the attribute interacts weirdly with optimizations spanning function calls, like inlining. It might be safe for TUs that define a single function, like with memmove above. Having said that applying the option to the TU itself would be equivalent, and not discouraged by GCC docs.
  - pertymcpert 2y ago
    Because these optimizations are not supposed to be a part of the user interface. They're internal passes and may one day go away or be merged or subsumed etc.
  - cozzyd 2y ago
    As far as I can tell it's due to some bugs

leni536 2y ago

"The optimize attribute should be used for debugging purposes only. It is not suitable in production code. " https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attribute...

Conscat 2y ago
That's an interesting note. I wonder why they claim this. As far as I know, `[[gnu::optimize("-fno-tree-loop-distribute-patterns")]]` (or the equivalent #pragma) is required for implementing a memcpy function in C unless you do something funky with the build system.
- leni536 2y ago
  Maybe that's applied to the TU that defines it? I don't see it in the glibc sources.
  - anarazel 2y ago
    It's indeed used: Definition of macro: https://sourceware.org/git/?p=glibc.git;a=blob;f=include/libc-symbols.h;h=4367aa6740bff6fc6b42b562f5d9a95b6690bc50;hb=HEAD#l808 https://sourceware.org/git/?p=glibc.git;a=blob;f=include/lib... Use: https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memmove.c;h=3fad5f11acdf5d8a5ae1436c1e577f050c5d5193;hb=HEAD#l42 https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memm... For a bunch of other places -fno-builtin-* seems to be used.
    - leni536 2y ago
      My guess is that the attribute interacts weirdly with optimizations spanning function calls, like inlining. It might be safe for TUs that define a single function, like with memmove above. Having said that applying the option to the TU itself would be equivalent, and not discouraged by GCC docs.
- pertymcpert 2y ago
  Because these optimizations are not supposed to be a part of the user interface. They're internal passes and may one day go away or be merged or subsumed etc.
- cozzyd 2y ago
  As far as I can tell it's due to some bugs