RE: [PATCH v3] x86: use builtins to read eflags

[David Laight]

From: Nick Desaulniers
> Sent: 07 February 2022 22:12
> 
> On Thu, Feb 3, 2022 at 4:57 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
> >
> > GCC and Clang both have builtins to read and write the EFLAGS register.
> > This allows the compiler to determine the best way to generate this
> > code, which can improve code generation.
> >
> > This issue arose due to Clang's issue with the "=rm" constraint.  Clang
> > chooses to be conservative in these situations, and so uses memory
> > instead of registers. This is a known issue, which is currently being
> > addressed.

How much performance would be lost by just using "=r"?

You get two instructions if the actual target is memory.
This might be a marginal code size increase - but not much,
It might also slow things down if the execution is limited
by the instruction decoder.

But on Intel cpu 'pop memory' is 2 uops, exactly the same
as 'pop register' 'store register' (and I think amd is similar).
So the actual execution time is exactly the same for both.

Also it looks like clang's builtin is effectively "=r".
Compiling:
long fl;
void native_save_fl(void) {
       fl = __builtin_ia32_readeflags_u64();
}
Not only generates a stack frame, it also generates:
pushf; pop %rax; mov mem, %rax.

	David

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