[PATCH] [InstCombine] Add a new formula for SMIN.

[Sanjoy Das]

Hi majnemer,

After this change `MatchSelectPattern` recognizes the following form
of SMIN:

  Y >s C ? ~Y : ~C == ~Y <s ~C ? ~Y : ~C = SMIN(~Y, ~C)

http://reviews.llvm.org/D9352

Files:
  lib/Transforms/InstCombine/InstCombineSelect.cpp
  test/Transforms/InstCombine/select.ll

Index: lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -86,6 +86,16 @@
         return (CmpLHS == FalseVal) ? SPF_ABS : SPF_NABS;
       }
     }
+
+    // Y >s C ? ~Y : ~C == ~Y <s ~C ? ~Y : ~C = SMIN(~Y, ~C)
+    if (ConstantInt *C2 = dyn_cast<ConstantInt>(FalseVal))
+      if (C1->getType() == C2->getType() && ~C1->getValue() == C2->getValue() &&
+          (match(TrueVal, m_Not(m_Specific(CmpLHS))) ||
+           match(CmpLHS, m_Not(m_Specific(TrueVal))))) {
+        LHS = TrueVal;
+        RHS = FalseVal;
+        return SPF_SMIN;
+      }
   }
 
   // TODO: (X > 4) ? X : 5   -->  (X >= 5) ? X : 5  -->  MAX(X, 5)
Index: test/Transforms/InstCombine/select.ll
===================================================================
--- test/Transforms/InstCombine/select.ll
+++ test/Transforms/InstCombine/select.ll
@@ -1520,3 +1520,15 @@
   %s1 = select i1 %c1, i32 %r0, i32 %s0
   ret i32 %s1
 }
+
+define i32 @test_max_of_min(i32 %a) {
+; MAX(MIN(%a, -1), -1) == -1
+; CHECK-LABEL: @test_max_of_min(
+; CHECK: ret i32 -1
+  %not_a = xor i32 %a, -1
+  %c0 = icmp sgt i32 %a, 0
+  %s0 = select i1 %c0, i32 %not_a, i32 -1
+  %c1 = icmp sgt i32 %s0, -1
+  %s1 = select i1 %c1, i32 %s0, i32 -1
+  ret i32 %s1
+}

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