[Transforms] Mirror optimizeStrRChr with optimizeStrChr

[AtariDreams]

strchr and strrchr have a lot in common. Basically, they can have most optimizations applied to them, only difference being we work backwards, and the "end" of the string being the first argument, and the "start" being the null terminator. Basically, we can do similar transformations with the same transformations and checks, especially if memrchr does the same as memchr but backwards from the null terminator.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

Changes

strchr and strrchr have a lot in common. Basically, they can have most optimizations applied to them, only difference being we work backwards, and the "end" of the string being the first argument, and the "start" being the null terminator. Basically, we can do similar transformations with the same transformations and checks, especially if memrchr does the same as memchr but backwards from the null terminator.

---

Full diff: https://github.com/llvm/llvm-project/pull/77685.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp (+46-12)

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index a7cd68e860e467..b79be5a7a4c5b5 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -483,8 +483,7 @@ Value *LibCallSimplifier::optimizeStrChr(CallInst *CI, IRBuilderBase &B) {
     Type *SizeTTy = IntegerType::get(CI->getContext(), SizeTBits);
     return copyFlags(*CI,
                      emitMemChr(SrcStr, CharVal, // include nul.
-                                ConstantInt::get(SizeTTy, Len), B,
-                                DL, TLI));
+                                ConstantInt::get(SizeTTy, Len), B, DL, TLI));
   }
 
   if (CharC->isZero()) {
@@ -523,22 +522,57 @@ Value *LibCallSimplifier::optimizeStrRChr(CallInst *CI, IRBuilderBase &B) {
   ConstantInt *CharC = dyn_cast<ConstantInt>(CharVal);
   annotateNonNullNoUndefBasedOnAccess(CI, 0);
 
+  if (!CharC) {
+    uint64_t Len = GetStringLength(SrcStr);
+    if (Len)
+      annotateDereferenceableBytes(CI, 0, Len);
+    else
+      return nullptr;
+
+    Function *Callee = CI->getCalledFunction();
+    FunctionType *FT = Callee->getFunctionType();
+    unsigned IntBits = TLI->getIntSize();
+    if (!FT->getParamType(1)->isIntegerTy(IntBits)) // memrchr needs 'int'.
+      return nullptr;
+
+    unsigned SizeTBits = TLI->getSizeTSize(*CI->getModule());
+    Type *SizeTTy = IntegerType::get(CI->getContext(), SizeTBits);
+
+    // Try to expand strrchr to the memrchr nonstandard extension if it's
+    // available, or simply fail otherwise.
+    return copyFlags(*CI,
+                     emitMemRChr(SrcStr, CharVal, // include nul.
+                                 ConstantInt::get(SizeTTy, Len), B, DL, TLI));
+  }
+
+  if (CharC->isZero()) {
+    Value *NullPtr = Constant::getNullValue(CI->getType());
+    if (isOnlyUsedInEqualityComparison(CI, NullPtr))
+      // Pre-empt the transformation to strlen below and fold
+      // strchr(A, '\0') == null to false.
+      return B.CreateIntToPtr(B.getTrue(), CI->getType());
+  }
+
+  // Otherwise, the character is a constant, see if the first argument is
+  // a string literal.  If so, we can constant fold.
   StringRef Str;
   if (!getConstantStringInfo(SrcStr, Str)) {
-    // strrchr(s, 0) -> strchr(s, 0)
-    if (CharC && CharC->isZero())
-      return copyFlags(*CI, emitStrChr(SrcStr, '\0', B, TLI));
+    if (CharC->isZero()) // strrchr(p, 0) -> p + strlen(p)
+      if (Value *StrLen = emitStrLen(SrcStr, B, DL, TLI))
+        return B.CreateInBoundsGEP(B.getInt8Ty(), SrcStr, StrLen, "strrchr");
     return nullptr;
   }
 
-  unsigned SizeTBits = TLI->getSizeTSize(*CI->getModule());
-  Type *SizeTTy = IntegerType::get(CI->getContext(), SizeTBits);
+  // Compute the offset, make sure to handle the case when we're searching for
+  // zero (a weird way to spell strlen).
+  size_t I = (0xFF & CharC->getSExtValue()) == 0
+                 ? Str.size()
+                 : Str.rfind(CharC->getSExtValue());
+  if (I == StringRef::npos) // Didn't find the char.  strrchr returns null.
+    return Constant::getNullValue(CI->getType());
 
-  // Try to expand strrchr to the memrchr nonstandard extension if it's
-  // available, or simply fail otherwise.
-  uint64_t NBytes = Str.size() + 1;   // Include the terminating nul.
-  Value *Size = ConstantInt::get(SizeTTy, NBytes);
-  return copyFlags(*CI, emitMemRChr(SrcStr, CharVal, Size, B, DL, TLI));
+  // strrchr(s+n,c)  -> gep(s+n+i,c)
+  return B.CreateInBoundsGEP(B.getInt8Ty(), SrcStr, B.getInt64(I), "strrchr");
 }
 
 Value *LibCallSimplifier::optimizeStrCmp(CallInst *CI, IRBuilderBase &B) {

[AtariDreams]

@MaskRay So the big question is: why would we return a poison value for strrchr but not strchr? why not let them have the same undefined behavior, or defined behavior (since we are allowed by the standard to have behavior for undefined behavior).