From 5e8cd99f5592ab80c27d8460054cd563cc8132ab Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 14 Jan 2015 19:26:56 +0000
Subject: [PATCH] InstCombine: Don't take A-B<0 into A<B if A-B has other uses

This fixes PR22226.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226023 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../InstCombine/InstCombineCompares.cpp       | 52 ++++++++++---------
 test/Transforms/InstCombine/icmp.ll           | 15 ++++++
 2 files changed, 42 insertions(+), 25 deletions(-)

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index f7075dfc674..c07c96d49aa 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2686,33 +2686,35 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         return Res;
     }
 
-    // (icmp ne/eq (sub A B) 0) -> (icmp ne/eq A, B)
-    if (I.isEquality() && CI->isZero() &&
-        match(Op0, m_Sub(m_Value(A), m_Value(B)))) {
-      // (icmp cond A B) if cond is equality
-      return new ICmpInst(I.getPredicate(), A, B);
+    // The following transforms are only 'worth it' if the only user of the
+    // subtraction is the icmp.
+    if (Op0->hasOneUse()) {
+      // (icmp ne/eq (sub A B) 0) -> (icmp ne/eq A, B)
+      if (I.isEquality() && CI->isZero() &&
+          match(Op0, m_Sub(m_Value(A), m_Value(B))))
+        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);
     }
 
-    // (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 ccb57d24451..64741c5712e 100644
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -1558,3 +1558,18 @@ define i1 @f4(i64 %a, i64 %b) {
   %v = icmp sle i64 %t, 0
   ret i1 %v
 }
+
+; CHECK-LABEL: @f5
+; CHECK: %[[cmp:.*]] = icmp slt i32 %[[sub:.*]], 0
+; CHECK: %[[neg:.*]] = sub nsw i32 0, %[[sub]]
+; CHECK: %[[sel:.*]] = select i1 %[[cmp]], i32 %[[neg]], i32 %[[sub]]
+; CHECK: ret i32 %[[sel]]
+define i32 @f5(i8 %a, i8 %b) {
+  %conv = zext i8 %a to i32
+  %conv3 = zext i8 %b to i32
+  %sub = sub nsw i32 %conv, %conv3
+  %cmp4 = icmp slt i32 %sub, 0
+  %sub7 = sub nsw i32 0, %sub
+  %sub7.sub = select i1 %cmp4, i32 %sub7, i32 %sub
+  ret i32 %sub7.sub
+}