Re: [patch 4/7] Immediate Values - i386 Optimization
From: H. Peter Anvin
Date: Sat Oct 20 2007 - 14:36:59 EST
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'.
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