From 1ac7c9979a2d723ff4649ffad58df02732872a5f Mon Sep 17 00:00:00 2001
From: Duncan Sands <baldrick@free.fr>
Date: Sun, 7 Nov 2010 16:12:23 +0000
Subject: [PATCH] Fix a README item: when doing a comparison with the result of
 a select instruction, see if doing the compare with the true and false values
 of the select gives the same result. If so, that can be used as the value of
 the comparison.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118378 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp  | 23 +++++++++++++++++++++--
 lib/Target/README.txt                 | 12 ------------
 test/Transforms/InstCombine/select.ll | 10 ++++++++++
 3 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index b49b4d0c6ab..1955802cc6e 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -252,8 +252,27 @@ Value *llvm::SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
       break;
     }
   }
-  
-  
+
+  // If the comparison is with the result of a select instruction, check whether
+  // comparing with either branch of the select always yields the same value.
+  if (isa<SelectInst>(LHS) || isa<SelectInst>(RHS)) {
+    // Make sure the select is on the LHS.
+    if (!isa<SelectInst>(LHS)) {
+      std::swap(LHS, RHS);
+      Pred = CmpInst::getSwappedPredicate(Pred);
+    }
+    SelectInst *SI = cast<SelectInst>(LHS);
+    // Now that we have "icmp select(cond, TV, FV), RHS", analyse it.
+    // Does "icmp TV, RHS" simplify?
+    if (Value *TCmp = SimplifyICmpInst(Pred, SI->getTrueValue(), RHS, TD))
+      // It does!  Does "icmp FV, RHS" simplify?
+      if (Value *FCmp = SimplifyICmpInst(Pred, SI->getFalseValue(), RHS, TD))
+        // It does!  If they simplified to the same value, then use it as the
+        // result of the original comparison.
+        if (TCmp == FCmp)
+          return TCmp;
+  }
+
   return 0;
 }
 
diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 55c08173d20..e63df536ae0 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1963,15 +1963,3 @@ bb3:            ; preds = %entry
         ret i32 %b
 }
 //===---------------------------------------------------------------------===//
-We should fold this code into "ret i1 false" since neither %zero nor %one can
-ever be null pointers.
-
-define i1 @foo(i1 %cond) {
-  %zero = alloca i32
-  %one = alloca i32
-
-  %ptr = select i1 %cond, i32* %zero, i32* %one
-  %isnull = icmp eq i32* %ptr, null
-  ret i1 %isnull
-}
-//===---------------------------------------------------------------------===//
diff --git a/test/Transforms/InstCombine/select.ll b/test/Transforms/InstCombine/select.ll
index 246a7bc5978..c641339cbb6 100644
--- a/test/Transforms/InstCombine/select.ll
+++ b/test/Transforms/InstCombine/select.ll
@@ -470,3 +470,13 @@ define i32 @test37(i32 %x) {
 ; CHECK: or i32 {{.*}}, 1
 ; CHECK: ret
 }
+
+define i1 @test38(i1 %cond) {
+  %zero = alloca i32
+  %one = alloca i32
+  %ptr = select i1 %cond, i32* %zero, i32* %one
+  %isnull = icmp eq i32* %ptr, null
+  ret i1 %isnull
+; CHECK: @test38
+; CHECK: ret i1 false
+}
-- 
2.34.1