[llvm] 5ba8020 - [DebugInfo][LSR] Emit shorter expressions from scev-based salvaging
Chris Jackson via llvm-commits
llvm-commits at lists.llvm.org
Sun Sep 19 13:42:14 PDT 2021
Author: Chris Jackson
Date: 2021-09-19T21:41:44+01:00
New Revision: 5ba8020326a522c0dfa32f59a472fe20bee4908a
URL: https://github.com/llvm/llvm-project/commit/5ba8020326a522c0dfa32f59a472fe20bee4908a
DIFF: https://github.com/llvm/llvm-project/commit/5ba8020326a522c0dfa32f59a472fe20bee4908a.diff
LOG: [DebugInfo][LSR] Emit shorter expressions from scev-based salvaging
The scev-based salvaging for LSR can sometimes produce unnecessarily
verbose expressions. This patch adds logic to detect when the value to
be recovered and the induction variable differ by only a constant
offset. Then, the expression to derive the current iteration count can
be omitted from the dbg.value in favour of the offset.
Reviewed by: aprantl
Differential Revision: https://reviews.llvm.org/D109044
Added:
Modified:
llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index a126c54d4248e..cbb8073256cfa 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -6126,13 +6126,13 @@ struct DVIRecoveryRec {
const llvm::SCEV *SCEV;
};
-static bool RewriteDVIUsingIterCount(DVIRecoveryRec CachedDVI,
+static void RewriteDVIUsingIterCount(DVIRecoveryRec CachedDVI,
const SCEVDbgValueBuilder &IterationCount,
ScalarEvolution &SE) {
// LSR may add locations to previously single location-op DVIs which
// are currently not supported.
if (CachedDVI.DVI->getNumVariableLocationOps() != 1)
- return false;
+ return;
// SCEVs for SSA values are most frquently of the form
// {start,+,stride}, but sometimes they are ({start,+,stride} + %a + ..).
@@ -6140,48 +6140,64 @@ static bool RewriteDVIUsingIterCount(DVIRecoveryRec CachedDVI,
// SCEVs have not been observed to result in debuginfo-lossy optimisations,
// so its not expected this point will be reached.
if (!isa<SCEVAddRecExpr>(CachedDVI.SCEV))
- return false;
+ return;
LLVM_DEBUG(dbgs() << "scev-salvage: Value to salvage SCEV: "
<< *CachedDVI.SCEV << '\n');
const auto *Rec = cast<SCEVAddRecExpr>(CachedDVI.SCEV);
if (!Rec->isAffine())
- return false;
+ return;
// Initialise a new builder with the iteration count expression. In
// combination with the value's SCEV this enables recovery.
SCEVDbgValueBuilder RecoverValue(IterationCount);
if (!RecoverValue.SCEVToValueExpr(*Rec, SE))
- return false;
+ return;
LLVM_DEBUG(dbgs() << "scev-salvage: Updating: " << *CachedDVI.DVI << '\n');
RecoverValue.applyExprToDbgValue(*CachedDVI.DVI, CachedDVI.Expr);
LLVM_DEBUG(dbgs() << "scev-salvage: to: " << *CachedDVI.DVI << '\n');
- return true;
}
-static bool
+static void RewriteDVIUsingOffset(DVIRecoveryRec &DVIRec, llvm::PHINode &IV,
+ int64_t Offset) {
+ assert(!DVIRec.DVI->hasArgList() && "Expected single location-op dbg.value.");
+ DbgValueInst *DVI = DVIRec.DVI;
+ SmallVector<uint64_t, 8> Ops;
+ DIExpression::appendOffset(Ops, Offset);
+ DIExpression *Expr = DIExpression::prependOpcodes(DVIRec.Expr, Ops, true);
+ LLVM_DEBUG(dbgs() << "scev-salvage: Updating: " << *DVIRec.DVI << '\n');
+ DVI->setExpression(Expr);
+ llvm::Value *ValIV = dyn_cast<llvm::Value>(&IV);
+ DVI->replaceVariableLocationOp(
+ 0u, llvm::MetadataAsValue::get(DVI->getContext(),
+ llvm::ValueAsMetadata::get(ValIV)));
+ LLVM_DEBUG(dbgs() << "scev-salvage: updated with offset to IV: "
+ << *DVIRec.DVI << '\n');
+}
+
+static void
DbgRewriteSalvageableDVIs(llvm::Loop *L, ScalarEvolution &SE,
llvm::PHINode *LSRInductionVar,
SmallVector<DVIRecoveryRec, 2> &DVIToUpdate) {
if (DVIToUpdate.empty())
- return false;
+ return;
const llvm::SCEV *SCEVInductionVar = SE.getSCEV(LSRInductionVar);
assert(SCEVInductionVar &&
"Anticipated a SCEV for the post-LSR induction variable");
- bool Changed = false;
if (const SCEVAddRecExpr *IVAddRec =
dyn_cast<SCEVAddRecExpr>(SCEVInductionVar)) {
if (!IVAddRec->isAffine())
- return false;
+ return;
+ // The iteratioun count is required to recover location values.
SCEVDbgValueBuilder IterCountExpr;
IterCountExpr.pushValue(LSRInductionVar);
if (!IterCountExpr.SCEVToIterCountExpr(*IVAddRec, SE))
- return false;
+ return;
LLVM_DEBUG(dbgs() << "scev-salvage: IV SCEV: " << *SCEVInductionVar
<< '\n');
@@ -6204,10 +6220,21 @@ DbgRewriteSalvageableDVIs(llvm::Loop *L, ScalarEvolution &SE,
DVIRec.DVI->setExpression(DVIRec.Expr);
}
- Changed |= RewriteDVIUsingIterCount(DVIRec, IterCountExpr, SE);
+ LLVM_DEBUG(dbgs() << "scev-salvage: value to recover SCEV: "
+ << *DVIRec.SCEV << '\n');
+
+ // Create a simple expression if the IV and value to salvage SCEVs
+ // start values
diff er by only a constant value.
+ if (Optional<APInt> Offset =
+ SE.computeConstantDifference(DVIRec.SCEV, SCEVInductionVar)) {
+ if (Offset.getValue().getMinSignedBits() <= 64)
+ RewriteDVIUsingOffset(DVIRec, *LSRInductionVar,
+ Offset.getValue().getSExtValue());
+ } else {
+ RewriteDVIUsingIterCount(DVIRec, IterCountExpr, SE);
+ }
}
}
- return Changed;
}
/// Identify and cache salvageable DVI locations and expressions along with the
diff --git a/llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll b/llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
index d5f2ef9383ef6..b9b725272d1b3 100644
--- a/llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
+++ b/llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
@@ -1,6 +1,8 @@
-; RUN: opt < %s -loop-reduce -S | FileCheck %s
+; RUN: opt -loop-reduce -S %s | FileCheck %s
-; Test that LSR preserves debug-info for induction variables.
+;; Test that LSR preserves debug-info for induction variables and scev-based
+;; salvaging produces short DIExpressions that use a constant offset from the
+;; induction variable.
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
@@ -21,11 +23,11 @@ for.body: ; preds = %entry, %for.body
call void @llvm.dbg.value(metadata i8 %i.06, metadata !14, metadata !DIExpression()), !dbg !17
call void @llvm.dbg.value(metadata i8* %p.addr.05, metadata !13, metadata !DIExpression()), !dbg !16
; CHECK-NOT: call void @llvm.dbg.value(metadata i8* undef
-; CHECK: call void @llvm.dbg.value(metadata !DIArgList(i8* %lsr.iv, i8* %p), metadata ![[MID_p:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_consts, 3, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_minus, DW_OP_consts, 3, DW_OP_div, DW_OP_consts, 3, DW_OP_mul, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_stack_value))
+; CHECK: all void @llvm.dbg.value(metadata i8* %lsr.iv, metadata ![[MID_p:[0-9]+]], metadata !DIExpression(DW_OP_constu, 3, DW_OP_minus, DW_OP_stack_value))
%add.ptr = getelementptr inbounds i8, i8* %p.addr.05, i64 3, !dbg !20
call void @llvm.dbg.value(metadata i8* %add.ptr, metadata !13, metadata !DIExpression()), !dbg !16
; CHECK-NOT: call void @llvm.dbg.value(metadata i8* undef
-; CHECK: call void @llvm.dbg.value(metadata !DIArgList(i8* %lsr.iv, i8* %p), metadata ![[MID_p]], metadata !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_consts, 3, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_minus, DW_OP_consts, 3, DW_OP_div, DW_OP_consts, 3, DW_OP_mul, DW_OP_consts, 3, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_plus, DW_OP_stack_value))
+; CHECK: call void @llvm.dbg.value(metadata i8* %lsr.iv, metadata ![[MID_p]], metadata !DIExpression())
store i8 %i.06, i8* %add.ptr, align 1, !dbg !23, !tbaa !24
%inc = add nuw nsw i8 %i.06, 1, !dbg !27
call void @llvm.dbg.value(metadata i8 %inc, metadata !14, metadata !DIExpression()), !dbg !17
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