[llvm] [Support][APint] Optimize fallback case in APInt::reverseBits (PR #189291)

[Max Graey via llvm-commits]

================
@@ -766,33 +766,43 @@ APInt APInt::byteSwap() const {
 }
 
 APInt APInt::reverseBits() const {
-  switch (BitWidth) {
-  case 64:
-    return APInt(BitWidth, llvm::reverseBits<uint64_t>(U.VAL));
-  case 32:
-    return APInt(BitWidth, llvm::reverseBits<uint32_t>(U.VAL));
-  case 16:
-    return APInt(BitWidth, llvm::reverseBits<uint16_t>(U.VAL));
-  case 8:
-    return APInt(BitWidth, llvm::reverseBits<uint8_t>(U.VAL));
-  case 0:
-    return *this;
-  default:
-    break;
-  }
-
-  APInt Val(*this);
-  APInt Reversed(BitWidth, 0);
-  unsigned S = BitWidth;
-
-  for (; Val != 0; Val.lshrInPlace(1)) {
-    Reversed <<= 1;
-    Reversed |= Val[0];
-    --S;
+  if (isSingleWord()) {
+    switch (BitWidth) {
+    case 64:
+      return APInt(BitWidth, llvm::reverseBits<uint64_t>(U.VAL));
+    case 32:
+      return APInt(BitWidth, llvm::reverseBits<uint32_t>(U.VAL));
+    case 16:
+      return APInt(BitWidth, llvm::reverseBits<uint16_t>(U.VAL));
+    case 8:
+      return APInt(BitWidth, llvm::reverseBits<uint8_t>(U.VAL));
+    case 1: // fallthrough
+    case 0:
+      return *this;
+    default:
+      return APInt(BitWidth,
+                   llvm::reverseBits<uint64_t>(U.VAL) >> (64 - BitWidth));
+    }
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MaxGraey wrote:

Then let's leave everything as it is for now

https://github.com/llvm/llvm-project/pull/189291