Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

[H. Peter Anvin]

On March 3, 2025 2:45:48 PM PST, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>On Mon, Mar 03, 2025 at 02:31:50PM -0800, H. Peter Anvin wrote:
>> >+#ifdef CONFIG_UNWINDER_FRAME_POINTER
>> > #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
>> >+#else
>> >+#define ASM_CALL_CONSTRAINT
>> >+#endif
>> > 
>> > #endif /* __ASSEMBLY__ */
>> > 
>> 
>> Wait, why was this changed? I actually tested this form at least once
>> and found that it didn't work under all circumstances...
>
>Do you have any more details about where this didn't work?  I tested
>with several configs and it seems to work fine.  Objtool will complain
>if it doesn't work.
>
>See here for the justification (the previous version was producing crap
>code in Clang):
>
>  https://lore.kernel.org/dbea2ae2fb39bece21013f939ddeb15507baa7d3.1740964309.git.jpoimboe@xxxxxxxxxx
>

I need to dig it up, but I had a discussion about this with some gcc people about a year ago.