[ConstraintElim] Use SCEV to check for multiples

[nikic]

When adding constraints for induction variables, if the step is not one, we need to make sure that (end-start) is a multiple of step, otherwise we might step over the end value.

Currently this only supports one specific pattern for pointers, where the end is a gep of the start with an appropriate offset.

Generalize this by using SCEV to check for multiples, which also makes this work for integer IVs.

There is no impact on compile-time.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

When adding constraints for induction variables, if the step is not one, we need to make sure that (end-start) is a multiple of step, otherwise we might step over the end value.

Currently this only supports one specific pattern for pointers, where the end is a gep of the start with an appropriate offset.

Generalize this by using SCEV to check for multiples, which also makes this work for integer IVs.

There is no impact on compile-time.

---

Full diff: https://github.com/llvm/llvm-project/pull/76925.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+4-17)
• (modified) llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-multiples.ll (+5-10)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index cc93c8617c05e2..5e57aa78175164 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -968,23 +968,10 @@ void State::addInfoForInductions(BasicBlock &BB) {
     return;
 
   if (!StepOffset.isOne()) {
-    auto *UpperGEP = dyn_cast<GetElementPtrInst>(B);
-    if (!UpperGEP || UpperGEP->getPointerOperand() != StartValue ||
-        !UpperGEP->isInBounds())
-      return;
-
-    MapVector<Value *, APInt> UpperVariableOffsets;
-    APInt UpperConstantOffset(StepOffset.getBitWidth(), 0);
-    const DataLayout &DL = BB.getModule()->getDataLayout();
-    if (!UpperGEP->collectOffset(DL, StepOffset.getBitWidth(),
-                                 UpperVariableOffsets, UpperConstantOffset))
-      return;
-    // All variable offsets and the constant offset have to be a multiple of the
-    // step.
-    if (!UpperConstantOffset.urem(StepOffset).isZero() ||
-        any_of(UpperVariableOffsets, [&StepOffset](const auto &P) {
-          return !P.second.urem(StepOffset).isZero();
-        }))
+    // Check whether B-Start is known to be a multiple of StepOffset.
+    const SCEV *BMinusStart = SE.getMinusSCEV(SE.getSCEV(B), StartSCEV);
+    if (isa<SCEVCouldNotCompute>(BMinusStart) ||
+        !SE.getConstantMultiple(BMinusStart).urem(StepOffset).isZero())
       return;
   }
 
diff --git a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-multiples.ll b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-multiples.ll
index 035cea4d73246c..a952c4048b450a 100644
--- a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-multiples.ll
+++ b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-multiples.ll
@@ -13,8 +13,7 @@ define void @multiple_pow2(i64 %count) {
 ; CHECK-NEXT:    [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_I_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP2_I_I:%.*]] = icmp ult i64 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP2_I_I]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -48,8 +47,7 @@ define void @multiple_pow2_larger_than_needed(i64 %count) {
 ; CHECK-NEXT:    [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_I_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP2_I_I:%.*]] = icmp ult i64 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP2_I_I]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -119,8 +117,7 @@ define void @multiple_pow2_start_offset(i64 %count) {
 ; CHECK-NEXT:    [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_I_NOT]], label [[EXIT]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP2_I_I:%.*]] = icmp ult i64 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP2_I_I]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -194,8 +191,7 @@ define void @multiple_pow2_start_offset_dynamic(i64 %count) {
 ; CHECK-NEXT:    [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_I_NOT]], label [[EXIT]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP2_I_I:%.*]] = icmp ult i64 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP2_I_I]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -231,8 +227,7 @@ define void @multiple_non_pow2_nuw(i64 %count) {
 ; CHECK-NEXT:    [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_I_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP2_I_I:%.*]] = icmp ult i64 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP2_I_I]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;

[fhahn]

LGTM, thanks!