[LLVMdev] strlen in fast-isel is missed

[Reed Kotler]

Seems like it might possible to override TargetLIbraryInfo during the 
creation of the fast-isel object.

I will look into this.

namespace llvm {
FastISel *Mips::createFastISel(FunctionLoweringInfo &funcInfo,
                                const TargetLibraryInfo *libInfo) {
   return new MipsFastISel(funcInfo, libInfo);
}
}


On 01/28/2015 11:03 AM, Reed Kotler wrote:
> This issue occurs in a many library functions.
> strlen, strcmp, ... many others.
> I'm surprised nobody has noticed this because many basic blocks will
> fail to be compiled as fast-isel in this case.
>
> It seems like just a bug in:
> bool FastISel::selectInstruction(const Instruction *I) {
>
> this function returns false but then it causes fast-isel to just do a
> miss on the function. there is no way to override the behavior.
>
> Since this is happening on a call instructtion, it's going to drop back
> and print as a missed fast isel call.
>
> It seems like this test in
> bool FastISel::selectInstruction(const Instruction *I)
>
> should just be deleted.
>
> Thoughts?
>
>
>      // As a special case, don't handle calls to builtin library
> functions that
>      // may be translated directly to target instructions.
>      if (F && !F->hasLocalLinkage() && F->hasName() &&
>          LibInfo->getLibFunc(F->getName(), Func) &&
>          LibInfo->hasOptimizedCodeGen(Func))
>        return false;
>
>
> This is called from void SelectionDAGISel::SelectAllBasicBlocks(const
> Function &Fn)
>
>      if (FastIS->selectInstruction(Inst)) {
>            --NumFastIselRemaining;
>            ++NumFastIselSuccess;
>            // If fast isel succeeded, skip over all the folded
> instructions, and
>            // then see if there is a load right before the selected
> instructions.
>            // Try to fold the load if so.
>            const Instruction *BeforeInst = Inst;
>            while (BeforeInst != Begin) {
>              BeforeInst =
> std::prev(BasicBlock::const_iterator(BeforeInst));
>              if (!isFoldedOrDeadInstruction(BeforeInst, FuncInfo))
>                break;
>            }
>            if (BeforeInst != Inst && isa<LoadInst>(BeforeInst) &&
>                BeforeInst->hasOneUse() &&
>                FastIS->tryToFoldLoad(cast<LoadInst>(BeforeInst), Inst)) {
>              // If we succeeded, don't re-select the load.
>              BI = std::next(BasicBlock::const_iterator(BeforeInst));
>              --NumFastIselRemaining;
>              ++NumFastIselSuccess;
>            }
>            continue;
>          }
>
> #ifndef NDEBUG
>          if (EnableFastISelVerbose2)
>            collectFailStats(Inst);
> #endif
>
>          // Then handle certain instructions as single-LLVM-Instruction
> blocks.
>          if (isa<CallInst>(Inst)) {
>
>            if (EnableFastISelVerbose || EnableFastISelAbort) {
>              dbgs() << "FastISel missed call: ";
>              Inst->dump();
>
>
>
> On 01/20/2015 02:35 PM, reed kotler wrote:
>> It seems that fast-isel for intel does not handle strlen. It's a general
>> problem in fast-isel .
>>
>>
>> ~/llvmw/build/Deb~/llvmw/build/Debug+Asserts/bin/clang -O0  -mllvm
>> -fast-isel-verbose -mllvm -fast-isel strlen1.c
>> strlen1.c:12:3: warning: implicitly declaring library function 'printf'
>> with
>>        type 'int (const char *, ...)'
>>    printf("%i\n", len);
>>    ^
>> strlen1.c:12:3: note: include the header <stdio.h> or explicitly
>> provide a
>>        declaration for 'printf'
>> FastISel missed call:   %call = call i64 @strlen(i8* %0) #3
>> 1 warning generated.
>>
>> #include <string.h>
>>
>> char *hello = "hello";
>> int len;
>>
>> void foo() {
>>    len = strlen(hello);
>> }
>>
>> int main() {
>>    foo();
>>    printf("%i\n", len);
>> }