[llvm-commits] [llvm] r53997 - in /llvm/trunk: lib/Transforms/Scalar/LICM.cpp test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll

[Dan Gohman]

Author: djg
Date: Thu Jul 24 18:57:25 2008
New Revision: 53997

URL: http://llvm.org/viewvc/llvm-project?rev=53997&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Transforms/Scalar/LICM.cpp
    llvm/trunk/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll

Modified: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LICM.cpp?rev=53997&r1=53996&r2=53997&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LICM.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp Thu Jul 24 18:57:25 2008
@@ -62,6 +62,15 @@
 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 @@
 
     // 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.

Modified: llvm/trunk/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll?rev=53997&r1=53996&r2=53997&view=diff

==============================================================================
--- llvm/trunk/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll (original)
+++ llvm/trunk/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll Thu Jul 24 18:57:25 2008
@@ -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) {