Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Sep 20, 2017 at 7:32 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 09/19/17 11:45, Josh Poimboeuf wrote:
>> For inline asm statements which have a CALL instruction, we list the
>> stack pointer as a constraint to convince GCC to ensure the frame
>> pointer is set up first:
>>
>>   static inline void foo()
>>   {
>>       register void *__sp asm(_ASM_SP);
>>       asm("call bar" : "+r" (__sp))
>>   }
>>
>> Unfortunately, that pattern causes clang to corrupt the stack pointer.
>>
>> There's actually an easier way to achieve the same goal in GCC, without
>> causing trouble for clang.  If we declare the stack pointer register
>> variable as a global variable, and remove the constraint altogether,
>> that convinces GCC to always set up the frame pointer before inserting
>> *any* inline asm.
>>
>> It basically acts as if *every* inline asm statement has a CALL
>> instruction.  It's a bit overkill, but the performance impact should be
>> negligible.
>>
>
> Again, probably negligible, but why do we need a frame pointer just
> because we have a call assembly instruction?

I think we need just the frame itself and RSP pointing below this
frame. If we don't have a frame, CALL instruction will smash whatever
RSP happens to point to. Compiler doesn't have to setup RSP to point
below used part of stack in leaf functions.