Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers
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- Subject: Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers
- From: "H. Peter Anvin" <hpa@xxxxxxxxx>
- Date: Mon, 03 Mar 2025 15:59:12 -0800
- Cc: linux-kernel@xxxxxxxxxxxxxxx, tip-bot2 for Josh Poimboeuf <tip-bot2@xxxxxxxxxxxxx>, linux-tip-commits@xxxxxxxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Brian Gerst <brgerst@xxxxxxxxx>, x86@xxxxxxxxxx
- In-reply-to: <20250303224548.pghzo2j4hdww7nxt@jpoimboe>
- References: <174099976188.10177.7153571701278544000.tip-bot2@tip-bot2> <C77024F6-3087-40A3-8AFB-A642EECAFF4E@zytor.com> <20250303224548.pghzo2j4hdww7nxt@jpoimboe>
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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.
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