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

From: H. Peter Anvin
Date: Wed Sep 20 2017 - 13:45:03 EST

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?