[LLVMdev] Which pass converts call printf to puts?

[Thomson]

Ok. So it seems CI->eraseFromParent() removed the old instruction and the
new one is inserted right after this one in the inner function in the case
of printf->puts. There is another line CI->repalceAllUsesWith(Result). I
think this line could also do the replacement besides inserting the new one
in the inner function.  What's the difference of these 2 replacement
methods?

Also thanks for your reminder of CC the mailing list.
-Thomson

On Mon, Jun 18, 2012 at 4:08 PM, Christoph Erhardt <christoph at sicherha.de>wrote:

> Hi Thomson,
>
> the new call to puts() is inserted right away, whereas the old call to
> printf() is removed a bit later in SimplifyLibCalls::runOnFunction(). If
> you browse the code a bit and backtrack the call stack to see what
> happens with the return value of PrintFOpt::OptimizeFixedFormatString(),
> you will stumble upon this segment in SimplifyLibCalls.cpp:1703ff.:
>
> // Try to optimize this call.
> Value *Result = LCO->OptimizeCall(CI, TD, TLI, Builder);
> if (Result == 0) continue;
>
> DEBUG(dbgs() << "SimplifyLibCalls simplified: " << *CI;
>      dbgs() << "  into: " << *Result << "\n");
>
> // Something changed!
> Changed = true;
> ++NumSimplified;
>
> // Inspect the instruction after the call (which was potentially just
> // added) next.
> I = CI; ++I;
>
> if (CI != Result && !CI->use_empty()) {
>  CI->replaceAllUsesWith(Result);
>  if (!Result->hasName())
>    Result->takeName(CI);
> }
> CI->eraseFromParent();
>
> Best regards,
> Christoph
>
> P.S. When answering, don't forget to CC the mailing list.
>
> On 18/06/2012 09:22, Thomson wrote:
> > Thanks for all your information. I got the pass in SimplifyLibCalls.cpp.
> >
> > I looked at the code, but am still a little confused about the IR
> > instruction replacement model. Use the following specific optimization
> > as example, it looks to me that even if a new call instruction (puts) is
> > created in EmitPutS, but the returned one is still the original one
> > (CI). So I am very curious about how the call instruction is replaced
> here.
> >
> >     // printf("%s\n", str) --> puts(str)
> >     if (FormatStr == "%s\n" && CI->getNumArgOperands() > 1 &&
> >         CI->getArgOperand(1)->getType()->isPointerTy()) {
> >       EmitPutS(CI->getArgOperand(1), B, TD);
> >       return CI;
> >     }
> >
> > -Thomson
>
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