[llvm] [InstCombine] Match a new form of truncating saturation (PR #189703)

[David Sherwood via llvm-commits]

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@@ -1171,6 +1171,33 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
     }
   }
 
+  // trunc (select(icmp_ult(A, DestTy_umax+1), A, sext(icmp_sgt(A, 0)))) -->
+  // trunc (smin(smax(0, A), DestTy_umax))
+  if (SrcTy->isIntegerTy() && isPowerOf2_64(SrcTy->getPrimitiveSizeInBits()) &&
+      DestTy->isIntegerTy() &&
+      isPowerOf2_64(DestTy->getPrimitiveSizeInBits()) &&
+      match(Src,
+            m_OneUse(m_Select(
+                m_SpecificICmp(ICmpInst::ICMP_ULT, m_Value(A), m_Constant(C)),
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david-arm wrote:

Should we also be checking for one use of both compares? Not sure if the transformation is worth it for multiple uses.

https://github.com/llvm/llvm-project/pull/189703