From 0f77ccd6bbe35543153dcd97a10fe552a5077e8d Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 31 Dec 2014 04:21:41 +0000
Subject: [PATCH] InstCombine: try to transform A-B < 0 into A < B

We are allowed to move the 'B' to the right hand side if we an prove
there is no signed overflow and if the comparison itself is signed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225034 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/InstCombine/InstCombineCompares.cpp | 20 ++++++++++++
 test/Transforms/InstCombine/icmp.ll                | 36 ++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 682820a25ca..381a687aca1 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2692,6 +2692,26 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
       return new ICmpInst(I.getPredicate(), A, B);
     }
 
+    // (icmp sgt (sub nsw A B), -1) -> (icmp sge A, B)
+    if (I.getPredicate() == ICmpInst::ICMP_SGT && CI->isAllOnesValue() &&
+        match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+      return new ICmpInst(ICmpInst::ICMP_SGE, A, B);
+
+    // (icmp sgt (sub nsw A B), 0) -> (icmp sgt A, B)
+    if (I.getPredicate() == ICmpInst::ICMP_SGT && CI->isZero() &&
+        match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+      return new ICmpInst(ICmpInst::ICMP_SGT, A, B);
+
+    // (icmp slt (sub nsw A B), 0) -> (icmp slt A, B)
+    if (I.getPredicate() == ICmpInst::ICMP_SLT && CI->isZero() &&
+        match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+      return new ICmpInst(ICmpInst::ICMP_SLT, A, B);
+
+    // (icmp slt (sub nsw A B), 1) -> (icmp sle A, B)
+    if (I.getPredicate() == ICmpInst::ICMP_SLT && CI->isOne() &&
+        match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+      return new ICmpInst(ICmpInst::ICMP_SLE, A, B);
+
     // If we have an icmp le or icmp ge instruction, turn it into the
     // appropriate icmp lt or icmp gt instruction.  This allows us to rely on
     // them being folded in the code below.  The SimplifyICmpInst code has
diff --git a/test/Transforms/InstCombine/icmp.ll b/test/Transforms/InstCombine/icmp.ll
index 279d86d4051..ccb57d24451 100644
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -1522,3 +1522,39 @@ define zeroext i1 @icmp_cmpxchg_strong(i32* %sc, i32 %old_val, i32 %new_val) {
   %icmp = icmp eq i32 %xtrc, %old_val
   ret i1 %icmp
 }
+
+; CHECK-LABEL: @f1
+; CHECK-NEXT: %[[cmp:.*]] = icmp sge i64 %a, %b
+; CHECK-NEXT: ret i1 %[[cmp]]
+define i1 @f1(i64 %a, i64 %b) {
+  %t = sub nsw i64 %a, %b
+  %v = icmp sge i64 %t, 0
+  ret i1 %v
+}
+
+; CHECK-LABEL: @f2
+; CHECK-NEXT: %[[cmp:.*]] = icmp sgt i64 %a, %b
+; CHECK-NEXT: ret i1 %[[cmp]]
+define i1 @f2(i64 %a, i64 %b) {
+  %t = sub nsw i64 %a, %b
+  %v = icmp sgt i64 %t, 0
+  ret i1 %v
+}
+
+; CHECK-LABEL: @f3
+; CHECK-NEXT: %[[cmp:.*]] = icmp slt i64 %a, %b
+; CHECK-NEXT: ret i1 %[[cmp]]
+define i1 @f3(i64 %a, i64 %b) {
+  %t = sub nsw i64 %a, %b
+  %v = icmp slt i64 %t, 0
+  ret i1 %v
+}
+
+; CHECK-LABEL: @f4
+; CHECK-NEXT: %[[cmp:.*]] = icmp sle i64 %a, %b
+; CHECK-NEXT: ret i1 %[[cmp]]
+define i1 @f4(i64 %a, i64 %b) {
+  %t = sub nsw i64 %a, %b
+  %v = icmp sle i64 %t, 0
+  ret i1 %v
+}
-- 
2.11.0