[tremor] [PATCH] 12% global performance gain on a StrongARM

[Nicolas Pitre]

On Thu, 19 Sep 2002, Chris Gilbert wrote:

> One thing I did notice, and it's not your bug, is that the CLIP_TO_15 is
> missing the fact that x is also an input arg, which means the compiler has
> no reason to assign x into the correct register, the fact that it works is
> probably luck (note I've not checked that it does work as is).

It's not buggy at all.  From GCC's manual:

* C Extensions::    GNU extensions to the C language family.
--> * Extended Asm::        Assembler instructions with C expressions as operands.
    --> *Note Modifiers::.

[...]
`='
     Means that this operand is write-only for this instruction: the
     previous value is discarded and replaced by output data.

`+'
     Means that this operand is both read and written by the
     instruction.

     When the compiler fixes up the operands to satisfy the constraints,
     it needs to know which operands are inputs to the instruction and
     which are outputs from it.  `=' identifies an output; `+'
     identifies an operand that is both input and output; all other
     operands are assumed to be input only.

> One thing that did puzzle me is why the memory barrier is needed in XPROD32,
> XPROD31 and XNPROD31, I guess I need to look at the asm, but doing *x = xl;
> the compiler should know that the memory has been updated.  AFAIK the memory
> barrier means that memory has been updated in some way that the compiler
> can't see, the need for the your change hints that the compiler is doing
> something wrong.

In some cases, the compiler would have inverted the two assignments.  
However *x really needs to be written to memory first since the value of y1 
is still being processed in the pipeline by the smlal instruction.  The 
memory barrier ensure that ordering.

<p>Nicolas

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