Re: [PATCH v2] x86: fix early boot crash on gcc-10

[Borislav Petkov]

On Wed, Apr 22, 2020 at 04:16:53PM +0200, Martin LiÅka wrote:
> And as I talked to Boris, I would recommend to come up with a "configure" check
> that a compiler does not optimize the key code sequence:
> 
> $ cat asm-detect.c
> int foo(int a);
> int bar(int a)
> {
>   int r = foo(a);
>   asm ("");
>   return r;
> }
> 
> $ gcc -O2 -c asm-detect.c -S -o/dev/stdout | grep jmp
> [no output]

That is a good test to run at the beginning of the compilation I guess.

Without the asm("") it produces:

bar:
.LFB0:
	.cfi_startproc
	jmp	foo@PLT
	.cfi_endproc

I'd like for LLVM folks to confirm that this is a good test for LLVM too
Trying that here with clang gives:

bar:                                    # @bar
        .cfi_startproc
# %bb.0:
        jmp     foo                     # TAILCALL
.Lfunc_end0:

so this *looks* like it would work with LLVM too but I might be missing
something...

Thx.

-- 
Regards/Gruss,
    Boris.

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