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Re: Some questions about pass web


For the relation between web and AUTOINC, I found the code in union_defs:

if (DF_REF_FLAGS (use) & DF_REF_READ_WRITE)
  {
    ...
  }

So web should handle AUTOINC in theory, but in an experiment, I added
a second web pass just before RA, web generated wrong code in the
presence of AUTOINC expressions. I will debug it more.

thanks
Guozhi Wei

On Tue, Sep 2, 2014 at 4:35 PM, Carrot Wei <carrot@google.com> wrote:
> Hi
>
> I have following questions about web (pseudo register renaming) pass:
>
> 1. It is well known that register renaming is a big help to register
> allocation, but in gcc's backend, the web pass is far before RA, there
> are about 20 passes between them. Does it mean register renaming can
> also heavily benefit other optimizations? And the passes between them
> usually don't generate more register renaming chances?
>
> 2. It looks current web pass can't handle AUTOINC expressions, a reg
> operand is used as both use and def reference in an AUTOINC
> expression, so this def side should not be renamed. Pass web doesn't
> explicitly check this case, may rename the reg operand of AUTOINC
> expression. Is this expected because it is before auto_inc_dec pass?
>
> 3. Are AUTOINC expressions only generated by auto_inc_dec pass? All
> passes before auto_inc_dec including expand should not generate
> AUTOINC expressions, otherwise it will break web.
>
> Could anybody help to answer these questions?
>
> thanks a lot
> Guozhi Wei


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