Re: Need some clarification about optimization flags, what "exactly" does -O1 do?

["Andrzej Giniewicz" <gginiu at gmail dot com>]

Hi,

> Compile with -fdump-tree-all.  You'll see dozens of dump files like
>
> hello.c.037t.addressables2
> hello.c.036t.dce1
> hello.c.035t.mergephi1
> hello.c.034t.copyprop1
> hello.c.033t.esra
> hello.c.032t.sdse1
> hello.c.031t.addressables1
> hello.c.030t.forwprop1
>
> Diff the pass before dce and the post-dce dump and you'll see what dce did.
> There are several dce passes.
>
> Also, you can diff the final optimized code with and without dce.

I did as you said, was trying to track it step by step at dce, but
finally checked how optimized version look... the code:

HardwareIndexBuffer::IndexType indexType =
mCurrentSection->get32BitIndices()?HardwareIndexBuffer::IT_32BIT :
HardwareIndexBuffer::IT_16BIT;

where IndexType is:

       enum IndexType {
         IT_16BIT,
         IT_32BIT
       };

and mCurrentSection is pointer field in current object of which
get32BitIndices() return m32BitIndices field, was optimized to:

indexType = (IndexType) this->mCurrentSection->m32BitIndices;

in above version the version without DCE had this in form of:

if (this->mCurrentSection->m32BitIndices) {
   indexType = 1;
} else {
  indexType = 0;
}

or exactly:

  D.288376 = this->mCurrentSection->m32BitIndices;
  if (D.288376)
    goto <bb 29>;
  else
    goto <bb 30>;

<bb 29>:
  indexType.4866 = 1;
  indexType = 1;
  goto <bb 31>;

<bb 30>:
  indexType.4867 = 0;
  indexType = 0;

I have not much experience, but it seems like it's not that bad...
maybe I should check how m32BitIndicies is initialized? (as reminder -
in optimized version indexType end to be 104 by some reason...)

cheers,
Andrzej.