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

[Mathieu Desnoyers]

* Jeremy Fitzhardinge (jeremy@xxxxxxxx) wrote:
> H. Peter Anvin wrote:
> > Allowing different registers should be doable, but if so, one would have
> > to put 0: at the *end* of the instruction and use (0f)-4 instead, since
> > the non-%eax forms are one byte longer.
> >   
> 
> OK, that's already a problem since its using "=r" as the constraint.
> 
> > This also seems "safer", since an imm32 is always the last thing in the
> > instruction.
> 
> Good idea.  If gas/gcc generates entirely the wrong addressing mode,
> then we've got bigger problems.
> 

Ok, let's have a good look at what we want:

1 - get a pointer to the beginning of the immediate value within the
    instruction.
2 - make sure that the immediate value, within the instruction, is
    written to atomically wrt all CPUs, even on older architectures
    where non aligned writes are not atomic.

Effectively, placing a label at the end of the instruction, and then
offsetting backward from there, will give us (1).

Then, for the alignment, we have to give a good look at the instruction
set reference, mov instruction, to see what the variants of mov
immediate value to register are on i386.

First, let's look at the possible prefixes:
- Lock and repeat prefixes : no
- Segment override prefixes: no memory reference there, so doesn't
  apply.
- Branch hints : no, it's a mov instruction
- Operand-size override prefix: *yes*, can be used for 2 bytes mov
- Address-size override prefix: no address in there, only immediate
  value and register.

(looking at the Compat/Leg Mode)

3 cases:

* 1 byte
B0 + rb         MOV r8, imm8     (1 byte opcode)
REX + B0 + rb   MOV r8, imm8     (only on 64 bits archs, never generated)
C6 /0           MOV r/m8, imm8   (2 bytes opcode)
(this one doesn't require alignment at all, since we do a 1 byte write)

* 2 bytes
B8 + rw         MOV r16, imm16   (1 byte opcode)
66 B8 + rd      MOV r16, imm16   (2 bytes opcode) (with 66H prefix)
C7 /0           MOV r/m16, imm16 (2 bytes opcode)
(Alignment on 4 bytes boundaries would be required because of the
possible 1 byte opcode ? Or is the 66H prefix mandatory there ? If it
is, then we can safely align on 2 bytes boundaries.)

* 4 bytes
B8 + rd         MOV r32, imm32   (1 byte opcode)
C7 /0           MOV r/m32, imm32 (2 bytes opcode)
(the 2 bytes opcode can be a problem)

I have missed anything ?

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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