[InstCombine] Fold icmp ugt/ult (udiv i32 C2, X), C1.

Chad Rosier · Chad Rosier · commit 4e6cda2db5d4 · 2016-05-10T20:22:09.000Z
This patch adds support for two optimizations: icmp ugt (udiv C2, X), C1 -> icmp ule X, C2/(C1+1) icmp ult (udiv C2, X), C1 -> icmp ugt X, C2/C1 Differential Revision: http://reviews.llvm.org/D20123 llvm-svn: 269109
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2093,8 +2093,28 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
     break;
   }
 
-  case Instruction::SDiv:
   case Instruction::UDiv:
+    if (ConstantInt *DivLHS = dyn_cast<ConstantInt>(LHSI->getOperand(0))) {
+      Value *X = LHSI->getOperand(1);
+      APInt C1 = RHS->getValue();
+      APInt C2 = DivLHS->getValue();
+      assert(C2 != 0 && "udiv 0, X should have been simplified already.");
+      // (icmp ugt (udiv C2, X), C1) -> (icmp ule X, C2/(C1+1))
+      if (ICI.getPredicate() == ICmpInst::ICMP_UGT) {
+        assert(!C1.isMaxValue() &&
+               "icmp ugt X, UINT_MAX should have been simplified already.");
+        return new ICmpInst(ICmpInst::ICMP_ULE, X,
+                            ConstantInt::get(X->getType(), C2.udiv(C1 + 1)));
+      }
+      // (icmp ult (udiv C2, X), C1) -> (icmp ugt X, C2/C1)
+      if (ICI.getPredicate() == ICmpInst::ICMP_ULT) {
+        assert(C1 != 0 && "icmp ult X, 0 should have been simplified already.");
+        return new ICmpInst(ICmpInst::ICMP_UGT, X,
+                            ConstantInt::get(X->getType(), C2.udiv(C1)));
+      }
+    }
+  // fall-through
+  case Instruction::SDiv:
     // Fold: icmp pred ([us]div X, C1), C2 -> range test
     // Fold this div into the comparison, producing a range check.
     // Determine, based on the divide type, what the range is being
@@ -2105,8 +2125,6 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
       if (Instruction *R = FoldICmpDivCst(ICI, cast<BinaryOperator>(LHSI),
                                           DivRHS))
         return R;
-    // FIXME: Handle (icmp ugt (udiv i32 CI2, A), CI) and
-    // (icmp ult (udiv i32 CI2, A), CI).
     break;
 
   case Instruction::Sub: {
diff --git a/llvm/test/Transforms/InstCombine/compare-udiv.ll b/llvm/test/Transforms/InstCombine/compare-udiv.ll
@@ -39,3 +39,94 @@ define i1 @test5(i32 %d) {
   %cmp1 = icmp ne i32 %div, 0
   ret i1 %cmp1
 }
+
+; CHECK-LABEL: @test6
+; CHECK: %cmp1 = icmp ult i32 %d, 6
+define i1 @test6(i32 %d) {
+  %div = udiv i32 5, %d
+  %cmp1 = icmp ugt i32 %div, 0
+  ret i1 %cmp1
+}
+
+; (icmp ugt (udiv C1, X), C1) -> false.
+; CHECK-LABEL: @test7
+; CHECK: ret i1 false
+define i1 @test7(i32 %d) {
+  %div = udiv i32 8, %d
+  %cmp1 = icmp ugt i32 %div, 8
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test8
+; CHECK: %cmp1 = icmp ult i32 %d, 2
+define i1 @test8(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ugt i32 %div, 3
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test9
+; CHECK: %cmp1 = icmp ult i32 %d, 2
+define i1 @test9(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ugt i32 %div, 2
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test10
+; CHECK: %cmp1 = icmp ult i32 %d, 3
+define i1 @test10(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ugt i32 %div, 1
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test11
+; CHECK: %cmp1 = icmp ugt i32 %d, 4
+define i1 @test11(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ult i32 %div, 1
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test12
+; CHECK: %cmp1 = icmp ugt i32 %d, 2
+define i1 @test12(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ult i32 %div, 2
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test13
+; CHECK: %cmp1 = icmp ugt i32 %d, 1
+define i1 @test13(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ult i32 %div, 3
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test14
+; CHECK: %cmp1 = icmp ugt i32 %d, 1
+define i1 @test14(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ult i32 %div, 4
+  ret i1 %cmp1
+}
+
+; icmp ugt X, UINT_MAX -> false.
+; CHECK-LABEL: @test15
+; CHECK: ret i1 false
+define i1 @test15(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ugt i32 %div, -1
+  ret i1 %cmp1
+}
+
+; icmp ult X, UINT_MAX -> true.
+; CHECK-LABEL: @test16
+; CHECK: ret i1 true
+define i1 @test16(i32 %d) {
+  %div = udiv i32 4, %d
+  %cmp1 = icmp ult i32 %div, -1
+  ret i1 %cmp1
+}
