[BuildLibCalls] emitPutChar should infer its function attributes

[craig.topper]

Summary

When InstCombine calls into SimplifyLibCalls and it createa putChar calls, we don't infer the attributes. And since SimplifyLibCalls doesn't use InstCombine's IRBuilder the calls doesn't end up in the worklist on this iteration of InstCombine. So it gets picked up on the next iteration where it causes an IR change. This of course causes InstCombine to run another iteration.

So this patch just gets the attributes right the first time. We already did this for puts and some other libcalls.

[spatel]

Can we do better than repeating this in every emit* function? Is there ever a case where we don't want to infer the attrs?

[craig.topper]

fputc and fputs check the type of one of the arguments before calling inferLibFuncAttributes. So its not always unconditional.

[spatel]

In D31094#704261, @craig.topper wrote:

fputc and fputs check the type of one of the arguments before calling inferLibFuncAttributes. So its not always unconditional.

In that case, LGTM.

[davide]

Can you add a testcase?

[craig.topper]

I'm not sure how to do that. We eventually inferred the attributes. It just took a second trip through the InstCombine worklist.

[efriedma]

You could use a debug counter to limit instcombine to one pass over the function; see http://llvm.org/docs/ProgrammersManual.html#adding-debug-counters-to-aid-in-debugging-your-code . Probably worth doing if you're planning to write more similar fixes.

llvm/trunk/lib/Transforms/Utils/BuildLibCalls.cpp

 Value *llvm::emitPutChar(Value *Char, IRBuilder<> &B,
                          const TargetLibraryInfo *TLI) {
   if (!TLI->has(LibFunc_putchar))
     return nullptr;
 
   Module *M = B.GetInsertBlock()->getModule();
   Value *PutChar = M->getOrInsertFunction("putchar", B.getInt32Ty(),
                                           B.getInt32Ty(), nullptr);
+  inferLibFuncAttributes(*M->getFunction("putchar"), *TLI);
   CallInst *CI = B.CreateCall(PutChar,
                               B.CreateIntCast(Char,
                               B.getInt32Ty(),
                               /*isSigned*/true,
                               "chari"),
                               "putchar");
 
   if (const Function *F = dyn_cast<Function>(PutChar->stripPointerCasts()))