On May 4, 2012, at 1:52 PM, Ian Lance Taylor wrote:
> <paul_kon...@dell.com> writes:
>
>> On May 4, 2012, at 11:39 AM, Ian Lance Taylor wrote:
>>
>>> <paul_kon...@dell.com> writes:
>>>
>>>> I thought that the "operand" in a mem:BLK is the pointer to the block,
>>>> not the block itself. So if the instruction(s) generated don't touch
>>>> the pointer -- a likely answer for a block-move instruction -- then
>>>> the operand would be read-only. Is that the right interpretation?
>>>
>>> Yes.
>>>
>>> But many block move instructions do in fact touch the pointer, in that
>>> they update the registers pointing to the starts of the blocks to point
>>> to the ends after the instruction completes.
>>
>> I interpreted + to mean that the operand is written with a value known to
>> the compiler, as opposed to clobber which means that the value is not known
>> (or not one that can be described to the compiler). So I take it that for
>> mem:BLK a + operand is interpreted as final value == end of the buffer? Or
>> byte after the buffer?
>
> Hmmmm. I don't really know what you mean, so there is some sort of
> communication difficulty.
>
> A '+' in a constraint for an operand means that the operand is both read
> and written by the instruction. It's relatively unusual to find such a
> constraint in a GCC backend. In a GCC backend, it's more common to
> write the instruction as a PARALLEL with one insn that sets the operand,
> another insn that uses the operand, and a matching constraint to put
> both operands in the same register.
>
> A '+' in a constraint doesn't say anything at all about what value the
> register has after the insn. That is expressed only in the RTL.
>
> The place where you often see '+' in a constraint is in an asm
> instruction. The asm instruction could also use a matching constraint,
> but there is generally less point since the asm instruction can't say
> anything about the value the register will have after the asm executes.
>
> Comparing '+' in a constraint and CLOBBER doesn't make sense. The '+'
> tells the register allocator something about which registers it may use.
> In particular, an operand with a '+' constraint may not be placed in a
> register that holds either an input or an output operand. An operand
> with an '=' constraint, on the other hand, may be placed in the same
> register as an input operand.
>
> Constraints like '+' matter to the register allocator and reload. RTL
> constructs like CLOBBER matter to the RTL optimizers. They are
> different categories of things.
>
> Ian
Thanks, that helps.
What I was trying to describe is the handling of a memcpy operation in the .md
file, where the operands are the memory pointers and (in my case) I want to
tell the machinery that the registers it's using to pass in the addresses no
longer have those addresses in them on completion. So I put in clobbers to say
that. What I really wanted to do is express that the pointer registers, on
completion, point just past the buffer, so the optimizer could take advantage
of that, but it wasn't clear how one would do that.
paul
