1 files changed, 44 insertions, 7 deletions
diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 40c1ba88354f1..55d3761c036f1 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -152,13 +152,50 @@ static bool matchSelectWithOptionalNotCond(Value *V, Value *&Cond, Value *&A,
     std::swap(A, B);
   }
 
-  // Set flavor if we find a match, or set it to unknown otherwise; in
-  // either case, return true to indicate that this is a select we can
-  // process.
-  if (auto *CmpI = dyn_cast<ICmpInst>(Cond))
-    Flavor = matchDecomposedSelectPattern(CmpI, A, B, A, B).Flavor;
-  else
-    Flavor = SPF_UNKNOWN;
+  // Match canonical forms of abs/nabs/min/max. We are not using ValueTracking's
+  // more powerful matchSelectPattern() because it may rely on instruction flags
+  // such as "nsw". That would be incompatible with the current hashing
+  // mechanism that may remove flags to increase the likelihood of CSE.
+
+  // These are the canonical forms of abs(X) and nabs(X) created by instcombine:
+  // %N = sub i32 0, %X
+  // %C = icmp slt i32 %X, 0
+  // %ABS = select i1 %C, i32 %N, i32 %X
+  //
+  // %N = sub i32 0, %X
+  // %C = icmp slt i32 %X, 0
+  // %NABS = select i1 %C, i32 %X, i32 %N
+  Flavor = SPF_UNKNOWN;
+  CmpInst::Predicate Pred;
+  if (match(Cond, m_ICmp(Pred, m_Specific(B), m_ZeroInt())) &&
+      Pred == ICmpInst::ICMP_SLT && match(A, m_Neg(m_Specific(B)))) {
+    // ABS: B < 0 ? -B : B
+    Flavor = SPF_ABS;
+    return true;
+  }
+  if (match(Cond, m_ICmp(Pred, m_Specific(A), m_ZeroInt())) &&
+      Pred == ICmpInst::ICMP_SLT && match(B, m_Neg(m_Specific(A)))) {
+    // NABS: A < 0 ? A : -A
+    Flavor = SPF_NABS;
+    return true;
+  }
+
+  if (!match(Cond, m_ICmp(Pred, m_Specific(A), m_Specific(B)))) {
+    // Check for commuted variants of min/max by swapping predicate.
+    // If we do not match the standard or commuted patterns, this is not a
+    // recognized form of min/max, but it is still a select, so return true.
+    if (!match(Cond, m_ICmp(Pred, m_Specific(B), m_Specific(A))))
+      return true;
+    Pred = ICmpInst::getSwappedPredicate(Pred);
+  }
+
+  switch (Pred) {
+  case CmpInst::ICMP_UGT: Flavor = SPF_UMAX; break;
+  case CmpInst::ICMP_ULT: Flavor = SPF_UMIN; break;
+  case CmpInst::ICMP_SGT: Flavor = SPF_SMAX; break;
+  case CmpInst::ICMP_SLT: Flavor = SPF_SMIN; break;
+  default: break;
+  }
 
   return true;
 }