diff options
author | Marco Elver <[email protected]> | 2023-08-11 17:18:38 +0200 |
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committer | Kees Cook <[email protected]> | 2023-08-15 14:57:24 -0700 |
commit | 7a0fd5e1678505534573b3c14c6ff69ed8592596 (patch) | |
tree | 75c5b189187af902999f7dbc4255f40bb5b2cd48 /scripts/gdb/linux/config.py | |
parent | 2e3f65ccfe6b0778b261ad69c9603ae85f210334 (diff) |
compiler_types: Introduce the Clang __preserve_most function attribute
[1]: "On X86-64 and AArch64 targets, this attribute changes the calling
convention of a function. The preserve_most calling convention attempts
to make the code in the caller as unintrusive as possible. This
convention behaves identically to the C calling convention on how
arguments and return values are passed, but it uses a different set of
caller/callee-saved registers. This alleviates the burden of saving and
recovering a large register set before and after the call in the caller.
If the arguments are passed in callee-saved registers, then they will be
preserved by the callee across the call. This doesn't apply for values
returned in callee-saved registers.
* On X86-64 the callee preserves all general purpose registers, except
for R11. R11 can be used as a scratch register. Floating-point
registers (XMMs/YMMs) are not preserved and need to be saved by the
caller.
* On AArch64 the callee preserve all general purpose registers, except
x0-X8 and X16-X18."
[1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
Introduce the attribute to compiler_types.h as __preserve_most.
Use of this attribute results in better code generation for calls to
very rarely called functions, such as error-reporting functions, or
rarely executed slow paths.
Beware that the attribute conflicts with instrumentation calls inserted
on function entry which do not use __preserve_most themselves. Notably,
function tracing which assumes the normal C calling convention for the
given architecture. Where the attribute is supported, __preserve_most
will imply notrace. It is recommended to restrict use of the attribute
to functions that should or already disable tracing.
Note: The additional preprocessor check against architecture should not
be necessary if __has_attribute() only returns true where supported;
also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until
__has_attribute() does the right thing, we also guard by known-supported
architectures to avoid build warnings on other architectures.
The attribute may be supported by a future GCC version (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
Signed-off-by: Marco Elver <[email protected]>
Reviewed-by: Miguel Ojeda <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Acked-by: "Steven Rostedt (Google)" <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>
Diffstat (limited to 'scripts/gdb/linux/config.py')
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