[SimplifyLibCalls] Remove over-eager early return in strlen optzns.

[fhahn]

Summary

Currently we bail out early for strlen calls with a GEP operand, if none
of the GEP specific optimizations fire. But there could be later
optimizations that still apply, which we currently miss out on.

An example is that we do not apply the following optimization

strlen(x) == 0 --> *x == 0

Unless I am missing something, there seems to be no reason for bailing
out early there.

Fixes PR47149.

[xbolva00]

LG

[lebedev.ri]

Not really familiar with the code, but that return does seem wrong.

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

if (getConstantDataArrayInfo(GEP->getOperand(0), Slice, CharSize)) {
       if ((Known.Zero.isNonNegative() && Known.Zero.ule(NullTermIdx)) ||
           (GEP->isInBounds() && isa<GlobalVariable>(GEP->getOperand(0)) &&
            NullTermIdx == ArrSize - 1)) {
         Offset = B.CreateSExtOrTrunc(Offset, CI->getType());
         return B.CreateSub(ConstantInt::get(CI->getType(), NullTermIdx),
                            Offset);
       }
     }
-
-    return nullptr;
   }
 
   // strlen(x?"foo":"bars") --> x ? 3 : 4
   if (SelectInst *SI = dyn_cast<SelectInst>(Src)) {
     uint64_t LenTrue = GetStringLength(SI->getTrueValue(), CharSize);
     uint64_t LenFalse = GetStringLength(SI->getFalseValue(), CharSize);
     if (LenTrue && LenFalse) {
       ORE.emit([&]() {

llvm/test/Transforms/InstCombine/strlen-1.ll

;
   %len = tail call i32 @strlen(i8* %str) nounwind
   ret i32 %len
 }
 
 ; Test cases for PR47149.
 define i1 @strlen0_after_write_to_first_byte_global() {
 ; CHECK-LABEL: @strlen0_after_write_to_first_byte_global(
 ; CHECK-NEXT:    store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), align 16
-; CHECK-NEXT:    [[LEN:%.*]] = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0))
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[LEN]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 0), align 16
   %len = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 0))
   %cmp = icmp eq i32 %len, 0
   ret i1 %cmp
 }
 
 define i1 @strlen0_after_write_to_second_byte_global() {
 ; CHECK-LABEL: @strlen0_after_write_to_second_byte_global(
 ; CHECK-NEXT:    store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 1), align 16
-; CHECK-NEXT:    [[LEN:%.*]] = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0))
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[LEN]], 0
+; CHECK-NEXT:    [[STRLENFIRST:%.*]] = load i8, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), align 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[STRLENFIRST]], 0
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 1), align 16
   %len = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 0))
   %cmp = icmp eq i32 %len, 0
   ret i1 %cmp
 }