[InstCombine] Add a new formula for SMIN.

[sanjoy]

Summary

After this change MatchSelectPattern recognizes the following form
of SMIN:

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

[majnemer]

Totally righteous, LGTM.

lib/Transforms/InstCombine/InstCombineSelect.cpp

91

Please brace this because it dominates a braced control-flow statement. Also, we can just use const auto *C2 here, the type is pretty clear.

lib/Transforms/InstCombine/InstCombineSelect.cpp

if ((CmpLHS == TrueVal && match(FalseVal, m_Neg(m_Specific(CmpLHS)))) ||
       }
 
       // ABS(X) ==> (X <s 0) ? -X : X and (X <s 1) ? -X : X
       // NABS(X) ==> (X <s 0) ? X : -X and (X <s 1) ? X : -X
       if (Pred == ICmpInst::ICMP_SLT && (C1->isZero() || C1->isOne())) {
         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)
 
   return SPF_UNKNOWN;
 }
 
 

test/Transforms/InstCombine/select.ll

define i32 @test_select_select1(i32 %a, i32 %r0, i32 %r1, i32 %v1, i32 %v2) {
   ; CHECK-NEXT: %[[SEL:.*]] = select i1 %[[C]], i32 %r0, i32 %r1
   ; CHECK-NEXT: ret i32 %[[SEL]]
   %c0 = icmp sge i32 %a, %v1
   %s0 = select i1 %c0, i32 %r0, i32 %r1
   %c1 = icmp slt i32 %a, %v2
   %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
+}