Re: [tip: x86/cleanups] x86/segment: Use MOVL when reading segment registers

[H. Peter Anvin]

On 2026-03-12 02:30, Uros Bizjak wrote:
> 
> so one can use this variant when the segment register is really saved
> to a memory. The above code improves to:
> 
>    32333:    8c a2 0c 0c 00 00        mov    %fs,0xc0c(%rdx)
>    32339:    8c aa 0e 0c 00 00        mov    %gs,0xc0e(%rdx)
> 
> Unfortunately, inline asm does not support alternative instructions
> that would depend on output argument type (reg vs mem), so the
> selection can not be automatic. But by having specialized macro, a
> developer can use the variant that results in the most optimal code.
> 
> A prototype patch is attached to the message.
> 

On a more general note, there is a *really* ugly way to distinguish operand
type on the gas side:


	.section ".rodata","a"

	.macro classify typevar:req argument:req
	\typevar = 0
	.irpc char,\argument
	.ifeq .L_type
	.ifc \char,%
	\typevar = 1		/* Register */
	.endif
	.ifc \char,(
	\typevar = 2		/* Memory */
	.endif
	.ifc \char,$
	\typevar = 3		/* Immediate */
	.endif
	.endif
	.endr
	.ifne .L_type
	\typevar = 2
	.endif
	.endm

	classify .L_type,%rax

	.ascii "Type = "
	.byte .L_type + 0x30
	.asciz "\n"


	-hpa