[InstCombine] Optimize icmp of inc/dec at RHS

Allow LLVM to optimize the sequence like the following:

  %inc = add nsw i32 %i, 1
  %cmp = icmp slt %n, %inc

into:

  %cmp = icmp sle i32 %n, %i

The case is not handled previously due to the complexity of compuation of %n.
Hence, LLVM cannot swap operands of icmp accordingly.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250746 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3426,6 +3426,26 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         match(B, m_One()))
       return new ICmpInst(CmpInst::ICMP_SGE, A, Op1);
 
+    // icmp sgt X, (Y + -1) -> icmp sge X, Y
+    if (C && NoOp1WrapProblem && Pred == CmpInst::ICMP_SGT &&
+        match(D, m_AllOnes()))
+      return new ICmpInst(CmpInst::ICMP_SGE, Op0, C);
+
+    // icmp sle X, (Y + -1) -> icmp slt X, Y
+    if (C && NoOp1WrapProblem && Pred == CmpInst::ICMP_SLE &&
+        match(D, m_AllOnes()))
+      return new ICmpInst(CmpInst::ICMP_SLT, Op0, C);
+
+    // icmp sge X, (Y + 1) -> icmp sgt X, Y
+    if (C && NoOp1WrapProblem && Pred == CmpInst::ICMP_SGE &&
+        match(D, m_One()))
+      return new ICmpInst(CmpInst::ICMP_SGT, Op0, C);
+
+    // icmp slt X, (Y + 1) -> icmp sle X, Y
+    if (C && NoOp1WrapProblem && Pred == CmpInst::ICMP_SLT &&
+        match(D, m_One()))
+      return new ICmpInst(CmpInst::ICMP_SLE, Op0, C);
+
     // if C1 has greater magnitude than C2:
     //  icmp (X + C1), (Y + C2) -> icmp (X + C3), Y
     //  s.t. C3 = C1 - C2

test/Transforms/InstCombine/icmp.ll

@@ -1632,3 +1632,43 @@ entry:
   %cmp580 = icmp ule i16 mul (i16 zext (i8 ptrtoint (i1 (i16)* @f10 to i8) to i16), i16 zext (i8 ptrtoint (i1 (i16)* @f10 to i8) to i16)), %p
   ret i1 %cmp580
 }
+
+; CHECK-LABEL: @cmp_sgt_rhs_dec
+; CHECK-NOT: sub
+; CHECK: icmp sge
+define i1 @cmp_sgt_rhs_dec(float %x, i32 %i) {
+  %conv = fptosi float %x to i32
+  %dec = sub nsw i32 %i, 1
+  %cmp = icmp sgt i32 %conv, %dec
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @cmp_sle_rhs_dec
+; CHECK-NOT: sub
+; CHECK: icmp slt
+define i1 @cmp_sle_rhs_dec(float %x, i32 %i) {
+  %conv = fptosi float %x to i32
+  %dec = sub nsw i32 %i, 1
+  %cmp = icmp sle i32 %conv, %dec
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @cmp_sge_rhs_inc
+; CHECK-NOT: add
+; CHECK: icmp sgt
+define i1 @cmp_sge_rhs_inc(float %x, i32 %i) {
+  %conv = fptosi float %x to i32
+  %inc = add nsw i32 %i, 1
+  %cmp = icmp sge i32 %conv, %inc
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @cmp_slt_rhs_inc
+; CHECK-NOT: add
+; CHECK: icmp sle
+define i1 @cmp_slt_rhs_inc(float %x, i32 %i) {
+  %conv = fptosi float %x to i32
+  %inc = add nsw i32 %i, 1
+  %cmp = icmp slt i32 %conv, %inc
+  ret i1 %cmp
+}