Re: [patch 4/7] Immediate Values - i386 Optimization

H. Peter Anvin
2007-10-20T18:36:57Z

Mathieu Desnoyers wrote:
>
> I have tried generating asm-to-"register" c variables for char, short
> and int on i386 and I do not see this happening. The char opcode is
> always 1 byte, short 2 bytes and int 1 byte. Result:
>

The comment was referring to x86-64, but I incorrectly remembered that
applying to "movq $imm,%reg" as opposed to loading from an absolute
address. gas actually has a special opcode (movabs) for the 64-bit
version of the latter variant, which is only available with %rax and its
subregisters.

Nevermind, in other words. It's still true, though, that the immediate
will always be the last thing in the instruction -- that's a fixture of
the instruction format.

> gcc version 4.1.3 20070812 (prerelease) (Debian 4.1.2-15)
>
> 8: b3 02 mov $0x2,%bl
> a: b1 03 mov $0x3,%cl
> c: b2 04 mov $0x4,%dl
> e: b0 05 mov $0x5,%al
>
> 4f: 66 be 06 00 mov $0x6,%si
> 53: 66 bb 07 00 mov $0x7,%bx
> 57: 66 b9 08 00 mov $0x8,%cx
> 5b: 66 ba 09 00 mov $0x9,%dx
> 5f: 66 b8 0a 00 mov $0xa,%ax
>
> 9f: bb 0b 00 00 00 mov $0xb,%ebx
> a4: be 0c 00 00 00 mov $0xc,%esi
> a9: b9 0d 00 00 00 mov $0xd,%ecx
> ae: ba 0e 00 00 00 mov $0xe,%edx
> b3: b8 0f 00 00 00 mov $0xf,%eax
>
>
> I notice that having a "=r" inline assembly that outputs to the first
> "register char" variable seems to be problematic. It fails with the
> following error:
>
> /tmp/ccy35Hq1.s: Assembler messages:
> /tmp/ccy35Hq1.s:15: Error: bad register name `%sil'

'r' is wrong for 8-bit variables on i386. It needs to be 'q'.