[llvm-commits] X86 FastISel: Emit immediate call arguments locally to save stack size when compiling with -O0

[Ivan Krasin]

On Thu, Aug 11, 2011 at 3:20 PM, Eric Christopher <echristo at apple.com> wrote:
>
> On Aug 10, 2011, at 4:01 PM, Ivan Krasin wrote:
>
> <flush-local-value-map.patch>
>
> In general I think the patch is OK.  A few requests:
>    MachineInstr *LastLocalValue;
> +  MachineInstr *OrigLocalValue;
> ….
>    // Start out as null, meaining no local-value instructions have
>    // been emitted.
> -  LastLocalValue = 0;
> +  OrigLocalValue = 0;
> A lot of the code you've changed hasn't had any comments written or updated
> for the new behavior. It'd be good to get an updated description of how the
> LocalValueMap is working and how it interacts with constants.
I have renamed OrigLocalValue to EmitStartPt which is (I believe) less
confusing naming, since it points to the place in the block where it's
allowed to start emitting instructions.

> In particular:
> +  if (!isa<IntrinsicInst>(F))
> +    flushLocalValueMap();
> +
> Here. The location of this here doesn't make a whole lot of sense and it'd
> be good if you could explain it.
I've added the explanation. Thanks for the suggestion.

> A better way to do this would be, as Jakob suggested, use the LocalValueMap
> as a storage for constants and locations that you've used them in the block
> and then emit all of the constants at that point rather than this weird
> flushing mechanism.
I've tried to do that. It slows down -O0 build which is unacceptable.
The problem is that we have to emit all local values to the start of
the block, store last use for each of them and spread them through the
block once the processing of the block is done. It makes it slower...
So, I would prefer to stay with the patch that makes all the metrics
slightly better on average (code size/stack size/compile time). Is it
fine with you?

Please, find the updated patch attached.

Ivan Krasin

> -eric
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