From 9feda1730c225589e144fb1f86a86a8cf3f39c9e Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Sat, 5 Mar 2011 04:28:48 +0000
Subject: [PATCH] Try once again to optimize "icmp (srem X, Y), Y" by turning
 the comparison into true/false or "icmp slt/sge Y, 0".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127063 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../InstCombine/InstCombineCompares.cpp       | 29 +++++++++++++++++++
 test/Transforms/InstCombine/icmp.ll           | 16 ++++++++++
 2 files changed, 45 insertions(+)

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index fe117c98143..239cd23a2c6 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2314,6 +2314,35 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         BO0->hasOneUse() && BO1->hasOneUse())
       return new ICmpInst(Pred, D, B);
 
+    BinaryOperator *SRem = NULL;
+    // icmp Y, (srem X, Y)
+    if (BO0 && BO0->getOpcode() == Instruction::SRem &&
+        Op1 == BO0->getOperand(1))
+      SRem = BO0;
+    // icmp (srem X, Y), Y
+    else if (BO1 && BO1->getOpcode() == Instruction::SRem &&
+             Op0 == BO1->getOperand(1))
+      SRem = BO1;
+    if (SRem) {
+      // We don't check hasOneUse to avoid increasing register pressure because
+      // the value we use is the same value this instruction was already using.
+      switch (SRem == BO0 ? ICmpInst::getSwappedPredicate(Pred) : Pred) {
+        default: break;
+        case ICmpInst::ICMP_EQ:
+          return ReplaceInstUsesWith(I, ConstantInt::getFalse(I.getContext()));
+        case ICmpInst::ICMP_NE:
+          return ReplaceInstUsesWith(I, ConstantInt::getTrue(I.getContext()));
+        case ICmpInst::ICMP_SGT:
+        case ICmpInst::ICMP_SGE:
+          return new ICmpInst(ICmpInst::ICMP_SGT, SRem->getOperand(1),
+                              Constant::getAllOnesValue(SRem->getType()));
+        case ICmpInst::ICMP_SLT:
+        case ICmpInst::ICMP_SLE:
+          return new ICmpInst(ICmpInst::ICMP_SLT, SRem->getOperand(1),
+                              Constant::getNullValue(SRem->getType()));
+      }
+    }
+
     if (BO0 && BO1 && BO0->getOpcode() == BO1->getOpcode() &&
         BO0->hasOneUse() && BO1->hasOneUse() &&
         BO0->getOperand(1) == BO1->getOperand(1)) {
diff --git a/test/Transforms/InstCombine/icmp.ll b/test/Transforms/InstCombine/icmp.ll
index fba273550b1..fb43e594535 100644
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -403,3 +403,19 @@ define i1 @test41(i32 %X, i32 %Y) {
   %B = icmp ugt i32 %Y, %A
   ret i1 %B
 }
+
+; CHECK: @test42
+; CHECK: %B = icmp sgt i32 %Y, -1
+define i1 @test42(i32 %X, i32 %Y) {
+  %A = srem i32 %X, %Y
+  %B = icmp slt i32 %A, %Y
+  ret i1 %B
+}
+
+; CHECK: @test43
+; CHECK: %B = icmp slt i32 %Y, 0
+define i1 @test43(i32 %X, i32 %Y) {
+  %A = srem i32 %X, %Y
+  %B = icmp slt i32 %Y, %A
+  ret i1 %B
+}