[clang-tools-extra] [InstCombine] Convert or concat to fshl if opposite or concat exists (PR #68502)

[via cfe-commits]

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@@ -2840,6 +2841,60 @@ static Instruction *matchFunnelShift(Instruction &Or, InstCombinerImpl &IC) {
       return nullptr;
 
     FShiftArgs = {ShVal0, ShVal1, ShAmt};
+  } else if (isa<ZExtInst>(Or0) || isa<ZExtInst>(Or1)) {
+    // If there are two 'or' instructions concat variables in opposite order:
+    //
+    // Slot1 and Slot2 are all zero bits.
+    // | Slot1 | Low | Slot2 | High |
+    // LowHigh = or (shl (zext Low), ZextLowShlAmt), (zext High)
+    // | Slot2 | High | Slot1 | Low |
+    // HighLow = or (shl (zext High), ZextHighShlAmt), (zext Low)
+    //
+    // the latter 'or' can be safely convert to
+    // -> HighLow = fshl LowHigh, LowHigh, ZextHighShlAmt
+    // if ZextLowShlAmt + ZextHighShlAmt == Width.
+    if (!isa<ZExtInst>(Or1))
+      std::swap(Or0, Or1);
+
+    Value *High, *ZextHigh, *Low;
+    const APInt *ZextHighShlAmt;
+    if (!match(Or0,
+               m_OneUse(m_Shl(m_Value(ZextHigh), m_APInt(ZextHighShlAmt)))))
+      return nullptr;
+
+    if (!match(Or1, m_ZExt(m_Value(Low))) ||
+        !match(ZextHigh, m_ZExt(m_Value(High))))
+      return nullptr;
+
+    unsigned HighSize = High->getType()->getScalarSizeInBits();
+    unsigned LowSize = Low->getType()->getScalarSizeInBits();
+    // Make sure High does not overlap with Low and most significant bits of
+    // High aren't shifted out.
+    if (ZextHighShlAmt->ult(LowSize) || ZextHighShlAmt->ugt(Width - HighSize))
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goldsteinn wrote:

Think you are missing a negative test for this.

https://github.com/llvm/llvm-project/pull/68502