[llvm] r270135 - [InstCombine] Avoid combining the bitcast of a var that is used as both address and result of load instructions

[Guozhi Wei via llvm-commits]

Author: carrot
Date: Thu May 19 16:07:01 2016
New Revision: 270135

URL: http://llvm.org/viewvc/llvm-project?rev=270135&view=rev
Log:
[InstCombine] Avoid combining the bitcast of a var that is used as both address and result of load instructions

This patch fixes https://llvm.org/bugs/show_bug.cgi?id=27703.

If there is a sequence of one or more load instructions, each loaded value is used as address of later load instruction, bitcast is necessary to change the value type, don't optimize it.


Added:
    llvm/trunk/test/Transforms/InstCombine/pr27703.ll
Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=270135&r1=270134&r2=270135&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Thu May 19 16:07:01 2016
@@ -1820,6 +1820,13 @@ Instruction *InstCombiner::optimizeBitCa
 
       auto *LI = dyn_cast<LoadInst>(IncValue);
       if (LI) {
+        // If there is a sequence of one or more load instructions, each loaded
+        // value is used as address of later load instruction, bitcast is
+        // necessary to change the value type, don't optimize it. For
+        // simplicity we give up if the load address comes from another load.
+        Value *Addr = LI->getOperand(0);
+        if (Addr == &CI || isa<LoadInst>(Addr))
+          return nullptr;
         if (LI->hasOneUse() && LI->isSimple())
           continue;
         // If a LoadInst has more than one use, changing the type of loaded

Added: llvm/trunk/test/Transforms/InstCombine/pr27703.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/pr27703.ll?rev=270135&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/pr27703.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/pr27703.ll Thu May 19 16:07:01 2016
@@ -0,0 +1,20 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+define void @mem() {
+bb:
+  br label %bb6
+
+bb6:
+  %.0 = phi i8** [ undef, %bb ], [ %t2, %bb6 ]
+  %tmp = load i8*, i8** %.0, align 8
+  %bc = bitcast i8* %tmp to i8**
+  %t1 = load i8*, i8** %bc, align 8
+  %t2 = bitcast i8* %t1 to i8**
+  br label %bb6
+
+bb206:
+  ret void
+; CHECK: phi
+; CHECK: bitcast
+; CHECK: load
+}