From: Dan Gohman <gohman@apple.com>
Date: Thu, 24 Jul 2008 23:57:25 +0000 (+0000)
Subject: Put the LICM of constant GlobalVariables, introduced in r53945, under a
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=3eee6542f5c65dce299361fa5435340513cf3fe4;p=oota-llvm.git

Put the LICM of constant GlobalVariables, introduced in r53945, under a
command-line option, and disable it by default. It introduced performance
regressions because CodeGen is currently not able to remat such loads.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53997 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/LICM.cpp b/lib/Transforms/Scalar/LICM.cpp
index 13440c13b65..aef1cd6f6f8 100644
--- a/lib/Transforms/Scalar/LICM.cpp
+++ b/lib/Transforms/Scalar/LICM.cpp
@@ -62,6 +62,15 @@ static cl::opt<bool>
 DisablePromotion("disable-licm-promotion", cl::Hidden,
                  cl::desc("Disable memory promotion in LICM pass"));
 
+// This feature is currently disabled by default because CodeGen is not yet capable
+// of rematerializing these constants in PIC mode, so it can lead to degraded
+// performance. Compile test/CodeGen/X86/remat-constant.ll with
+// -relocation-model=pic to see an example of this.
+static cl::opt<bool>
+EnableLICMConstantMotion("enable-licm-constant-variables", cl::Hidden,
+                         cl::desc("Enable hoisting/sinking of constant "
+                                  "global variables"));
+
 namespace {
   struct VISIBILITY_HIDDEN LICM : public LoopPass {
     static char ID; // Pass identification, replacement for typeid
@@ -372,7 +381,8 @@ bool LICM::canSinkOrHoistInst(Instruction &I) {
 
     // Loads from constant memory are always safe to move, even if they end up
     // in the same alias set as something that ends up being modified.
-    if (AA->pointsToConstantMemory(LI->getOperand(0)))
+    if (EnableLICMConstantMotion &&
+        AA->pointsToConstantMemory(LI->getOperand(0)))
       return true;
     
     // Don't hoist loads which have may-aliased stores in loop.
diff --git a/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll b/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
index c3af29bf436..3824d535977 100644
--- a/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
+++ b/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
@@ -1,4 +1,4 @@
-; RUN: llvm-as < %s | opt -licm | llvm-dis | grep -A 1 entry | grep load.*@a
+; RUN: llvm-as < %s | opt -licm -enable-licm-constant-variables | llvm-dis | grep -A 1 entry | grep load.*@a
 @a = external constant float*
 
 define void @test(i32 %count) {