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Put the LICM of constant GlobalVariables, introduced in r53945, under a

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command-line option, and disable it by default. It introduced performance
regressions because CodeGen is currently not able to remat such loads.

llvm-svn: 53997
This commit is contained in:
Dan Gohman 2008-07-24 23:57:25 +00:00
parent da5c2b50b8
commit 1ecbcecdf3
2 changed files with 12 additions and 2 deletions
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12
lib/Transforms/Scalar/LICM.cpp
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@ -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.

2
test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
View File

@ -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) {