[CVP] Don't try to fold load/store operands to constant

[nikic]

CVP currently tries to fold load/store pointer operands to constants using LVI. If there is a dominating condition of the form icmp eq ptr %p, @g, then %p will be replaced with @g.

LVI is geared towards range-based optimizations, and is very inefficient at handling simple pointer equality conditions. We have other passes that can handle this optimization in a more efficient way, such as IPSCCP and GVN.

Removing this optimization gives a geomean 0.4-1.2% compile-time improvement depending on configuration: http://llvm-compile-time-tracker.com/compare.php?from=7eeedc124f9901a65573668bc504a45111a3f837&to=4769e20ad4d5f35db561bf14b00e91cec325039a&stat=instructions:u Especially notable is a 4% improvement on tramp3d-v4 with ThinLTO. At the same time, there is no impact on codegen.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

CVP currently tries to fold load/store pointer operands to constants using LVI. If there is a dominating condition of the form icmp eq ptr %p, @<!-- -->g, then %p will be replaced with @<!-- -->g.

LVI is geared towards range-based optimizations, and is very inefficient at handling simple pointer equality conditions. We have other passes that can handle this optimization in a more efficient way, such as IPSCCP and GVN.

Removing this optimization gives a geomean 0.4-1.2% compile-time improvement depending on configuration: http://llvm-compile-time-tracker.com/compare.php?from=7eeedc124f9901a65573668bc504a45111a3f837&to=4769e20ad4d5f35db561bf14b00e91cec325039a&stat=instructions:u Especially notable is a 4% improvement on tramp3d-v4 with ThinLTO. At the same time, there is no impact on codegen.

---

Full diff: https://github.com/llvm/llvm-project/pull/73338.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp (-22)
• (modified) llvm/test/Transforms/CorrelatedValuePropagation/basic.ll (+1-1)

diff --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 3229ec4f4d0f52e..1bdc441d18ea65e 100644
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -55,7 +55,6 @@ static cl::opt<bool> CanonicalizeICmpPredicatesToUnsigned(
 STATISTIC(NumPhis,      "Number of phis propagated");
 STATISTIC(NumPhiCommon, "Number of phis deleted via common incoming value");
 STATISTIC(NumSelects,   "Number of selects propagated");
-STATISTIC(NumMemAccess, "Number of memory access targets propagated");
 STATISTIC(NumCmps,      "Number of comparisons propagated");
 STATISTIC(NumReturns,   "Number of return values propagated");
 STATISTIC(NumDeadCases, "Number of switch cases removed");
@@ -264,23 +263,6 @@ static bool processPHI(PHINode *P, LazyValueInfo *LVI, DominatorTree *DT,
   return Changed;
 }
 
-static bool processMemAccess(Instruction *I, LazyValueInfo *LVI) {
-  Value *Pointer = nullptr;
-  if (LoadInst *L = dyn_cast<LoadInst>(I))
-    Pointer = L->getPointerOperand();
-  else
-    Pointer = cast<StoreInst>(I)->getPointerOperand();
-
-  if (isa<Constant>(Pointer)) return false;
-
-  Constant *C = LVI->getConstant(Pointer, I);
-  if (!C) return false;
-
-  ++NumMemAccess;
-  I->replaceUsesOfWith(Pointer, C);
-  return true;
-}
-
 static bool processICmp(ICmpInst *Cmp, LazyValueInfo *LVI) {
   if (!CanonicalizeICmpPredicatesToUnsigned)
     return false;
@@ -1159,10 +1141,6 @@ static bool runImpl(Function &F, LazyValueInfo *LVI, DominatorTree *DT,
       case Instruction::FCmp:
         BBChanged |= processCmp(cast<CmpInst>(&II), LVI);
         break;
-      case Instruction::Load:
-      case Instruction::Store:
-        BBChanged |= processMemAccess(&II, LVI);
-        break;
       case Instruction::Call:
       case Instruction::Invoke:
         BBChanged |= processCallSite(cast<CallBase>(II), LVI);
diff --git a/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll b/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
index 0ef48ea8020ec30..9fcf7c320f62dd5 100644
--- a/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
@@ -63,7 +63,7 @@ define i8 @test3(ptr %a) nounwind {
 ; CHECK:       bb:
 ; CHECK-NEXT:    ret i8 0
 ; CHECK:       bb2:
-; CHECK-NEXT:    [[SHOULD_BE_CONST:%.*]] = load i8, ptr @gv, align 1
+; CHECK-NEXT:    [[SHOULD_BE_CONST:%.*]] = load i8, ptr [[A]], align 1
 ; CHECK-NEXT:    ret i8 [[SHOULD_BE_CONST]]
 ;
 entry:

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 5e5a22caf88ac1ccfa8dc5720295fdeba0ad9372 b82eb1e065cb20f3438928f6a2806d3a511785ee -- llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 1bdc441d18..dfb64d88b4 100644
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -54,7 +54,7 @@ static cl::opt<bool> CanonicalizeICmpPredicatesToUnsigned(
 
 STATISTIC(NumPhis,      "Number of phis propagated");
 STATISTIC(NumPhiCommon, "Number of phis deleted via common incoming value");
-STATISTIC(NumSelects,   "Number of selects propagated");
+STATISTIC(NumSelects, "Number of selects propagated");
 STATISTIC(NumCmps,      "Number of comparisons propagated");
 STATISTIC(NumReturns,   "Number of return values propagated");
 STATISTIC(NumDeadCases, "Number of switch cases removed");

[fhahn]

LGTM! Looks like this may need clang-formatting before landing.