[CVP] Remove Zero-Index GEP #144831

veera-sivarajan · 2025-06-19T04:00:31Z

Fixes: #137168

Proof: https://alive2.llvm.org/ce/z/Xoyiar

Replaces all uses of a GEP with its base pointer when all its indices are known to be in the range [0, 1).

This is the same transform as 87a0b1b but in CVP.

llvmbot · 2025-06-19T04:01:03Z

@llvm/pr-subscribers-llvm-transforms

Author: Veera (veera-sivarajan)

Changes

Fixes: #137168

Proof: https://alive2.llvm.org/ce/z/Xoyiar

Replaces all uses of a GEP with its base pointer when all its indices are known to be in the range [0, 1).

This is the same transform as 87a0b1b but in CVP.

Full diff: https://github.com/llvm/llvm-project/pull/144831.diff

2 Files Affected:

(modified) llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp (+21)
(added) llvm/test/Transforms/CorrelatedValuePropagation/gep.ll (+45)

diff --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 4627f537dc16b..11a3a35d1ae7c 100644
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -47,6 +47,7 @@ using namespace llvm;
 
 #define DEBUG_TYPE "correlated-value-propagation"
 
+STATISTIC(NumGEPs,      "Number of geps propagated");
 STATISTIC(NumPhis,      "Number of phis propagated");
 STATISTIC(NumPhiCommon, "Number of phis deleted via common incoming value");
 STATISTIC(NumSelects,   "Number of selects propagated");
@@ -1252,6 +1253,23 @@ static bool processTrunc(TruncInst *TI, LazyValueInfo *LVI) {
   return Changed;
 }
 
+// remove a GEP if all indices are in range [0, 1)
+static bool processGEP(GetElementPtrInst *GEP, LazyValueInfo *LVI) {
+  for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) {
+    ConstantRange Range = LVI->getConstantRangeAtUse(GEP->getOperandUse(i),
+                                                     /*UndefAllowed=*/false);
+    const APInt *C = Range.getSingleElement();
+    if (!C || !C->isZero())
+      return false;
+  }
+
+  // all indices are zero
+  Value *Ptr = GEP->getPointerOperand();
+  GEP->replaceAllUsesWith(Ptr);
+  ++NumGEPs;
+  return true;
+}
+
 static bool runImpl(Function &F, LazyValueInfo *LVI, DominatorTree *DT,
                     const SimplifyQuery &SQ) {
   bool FnChanged = false;
@@ -1318,6 +1336,9 @@ static bool runImpl(Function &F, LazyValueInfo *LVI, DominatorTree *DT,
       case Instruction::Trunc:
         BBChanged |= processTrunc(cast<TruncInst>(&II), LVI);
         break;
+      case Instruction::GetElementPtr:
+        BBChanged |= processGEP(cast<GetElementPtrInst>(&II), LVI);
+        break;
       }
     }
 
diff --git a/llvm/test/Transforms/CorrelatedValuePropagation/gep.ll b/llvm/test/Transforms/CorrelatedValuePropagation/gep.ll
new file mode 100644
index 0000000000000..d5c122b3f665e
--- /dev/null
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/gep.ll
@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=correlated-propagation -S | FileCheck %s
+
+define ptr @noop_gep(ptr %ptr, i8 range(i8 0, 1) %index) {
+; CHECK-LABEL: define ptr @noop_gep(
+; CHECK-SAME: ptr [[PTR:%.*]], i8 range(i8 0, 1) [[INDEX:%.*]]) {
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i1, ptr [[PTR]], i8 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %gep = getelementptr i1, ptr %ptr, i8 %index
+  ret ptr %gep
+}
+
+
+define ptr @noop_gep_with_constant(ptr %ptr, i8 range(i8 0, 1) %index) {
+; CHECK-LABEL: define ptr @noop_gep_with_constant(
+; CHECK-SAME: ptr [[PTR:%.*]], i8 range(i8 0, 1) [[INDEX:%.*]]) {
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr [2 x i1], ptr [[PTR]], i8 0, i8 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %gep = getelementptr [2 x i1], ptr %ptr, i8 0, i8 %index
+  ret ptr %gep
+}
+
+
+define ptr @noop_gep_nonzero_index_negative(ptr %ptr, i8 range(i8 0, 2) %index) {
+; CHECK-LABEL: define ptr @noop_gep_nonzero_index_negative(
+; CHECK-SAME: ptr [[PTR:%.*]], i8 range(i8 0, 2) [[INDEX:%.*]]) {
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i1, ptr [[PTR]], i8 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+  %gep = getelementptr i1, ptr %ptr, i8 %index
+  ret ptr %gep
+}
+
+
+define ptr @gep_no_range_negative(ptr %ptr, i8 %index) {
+; CHECK-LABEL: define ptr @gep_no_range_negative(
+; CHECK-SAME: ptr [[PTR:%.*]], i8 [[INDEX:%.*]]) {
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i1, ptr [[PTR]], i8 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+  %gep = getelementptr i1, ptr %ptr, i8 %index
+  ret ptr %gep
+}

github-actions · 2025-06-19T04:02:47Z

✅ With the latest revision this PR passed the C/C++ code formatter.

dtcxzyw · 2025-06-19T06:06:08Z

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

+
+  // all indices are zero
+  Value *Ptr = GEP->getPointerOperand();
+  GEP->replaceAllUsesWith(Ptr);


It doesn't work for the form getelementptr ty, scalar ptr, vector indices. You can create a splat when the type of GEP differs from the type of source pointer.

Crash reproducer:

define <2 x ptr> @noop_gep_nonzero_index_vec(ptr %ptr, <2 x i64> range(i64 0, 1) %index) { %gep = getelementptr i8, ptr %ptr, <2 x i64> %index ret <2 x ptr> %gep }

dtcxzyw · 2025-06-19T06:07:37Z

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -1252,6 +1253,23 @@ static bool processTrunc(TruncInst *TI, LazyValueInfo *LVI) {
  return Changed;
 }

+// remove a GEP if all indices are in range [0, 1)


Suggested change

// remove a GEP if all indices are in range [0, 1)

/// Remove a GEP if all indices are in range [0, 1)

dtcxzyw · 2025-06-19T06:08:12Z

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -1252,6 +1253,23 @@ static bool processTrunc(TruncInst *TI, LazyValueInfo *LVI) {
  return Changed;
 }

+// remove a GEP if all indices are in range [0, 1)
+static bool processGEP(GetElementPtrInst *GEP, LazyValueInfo *LVI) {
+  for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) {


Suggested change

for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) {

for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I) {

dtcxzyw · 2025-06-19T06:09:37Z

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

+  // all indices are zero
+  Value *Ptr = GEP->getPointerOperand();
+  GEP->replaceAllUsesWith(Ptr);
+  ++NumGEPs;


Suggested change

++NumGEPs;

GEP->eraseFromParent();

++NumGEPs;

dtcxzyw · 2025-06-19T06:10:40Z

llvm/test/Transforms/CorrelatedValuePropagation/gep.ll

+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i1, ptr [[PTR]], i8 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %gep = getelementptr i1, ptr %ptr, i8 %index


Suggested change

%gep = getelementptr i1, ptr %ptr, i8 %index

%gep = getelementptr i8, ptr %ptr, i64 %index

Use i8 as the pointee type and i64 as the index type.

dtcxzyw · 2025-06-19T06:11:47Z

llvm/test/Transforms/CorrelatedValuePropagation/gep.ll

@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=correlated-propagation -S | FileCheck %s
+


Can you add the motivating case from #137168?

nikic

There is nothing really GEP specific here -- this just uses LVI to determine whether values are constant. However, there is a reason why we don't actually do this very obvious optimization: Unlike SCCP (which does perform this type of optimization), LVI is very, very slow and we need to keep the number of values we query small.

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=a9a71b6d311892d6add6aab3790b20fe945cca38&to=f4918da327f8d1cb43fb6aa181c948f0f9e37979&stat=instructions:u

nikic · 2025-06-19T08:25:27Z

A possible way to fix the motivating issue is to enable use of PredicateInfo in non-IP SCCP. This will also have compile-time overhead, but probably less than this? We'd have to evaluate whether it is worthwhile.

Edit: PredicateInfo compile-time: https://llvm-compile-time-tracker.com/compare.php?from=a9a71b6d311892d6add6aab3790b20fe945cca38&to=e237b4a75e0eadb116cec3fa3227f7f2e95ed996&stat=instructions:u

Add Test

e8650a0

veera-sivarajan requested a review from nikic as a code owner June 19, 2025 04:00

llvmbot added the llvm:transforms label Jun 19, 2025

veera-sivarajan requested a review from dtcxzyw June 19, 2025 04:00

[CVP] Remove Zero-Index GEP

4cdb154

veera-sivarajan force-pushed the fix-137168 branch from a014cef to 4cdb154 Compare June 19, 2025 04:05

dtcxzyw reviewed Jun 19, 2025

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[CVP] Remove Zero-Index GEP #144831

[CVP] Remove Zero-Index GEP #144831

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nikic commented Jun 19, 2025 •

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	// remove a GEP if all indices are in range [0, 1)
	/// Remove a GEP if all indices are in range [0, 1)

	for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) {
	for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I) {

	%gep = getelementptr i1, ptr %ptr, i8 %index
	%gep = getelementptr i8, ptr %ptr, i64 %index

		@@ -0,0 +1,45 @@
		; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
		; RUN: opt < %s -passes=correlated-propagation -S \| FileCheck %s

[CVP] Remove Zero-Index GEP #144831

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[CVP] Remove Zero-Index GEP #144831

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