[PATCH] D109044: [DebugInfo][LoopStrengthReduction] Produce shorter expressions when using scev-based saslvaging

[Chris Jackson via Phabricator via llvm-commits]

chrisjackson updated this revision to Diff 369864.
chrisjackson added a comment.

Rebase.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D109044/new/

https://reviews.llvm.org/D109044

Files:
  llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
  llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll


Index: llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
===================================================================
--- llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-0.ll
+++ 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 @@
   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
Index: llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -6161,6 +6161,25 @@
   return true;
 }
 
+static bool 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 *OldExpr =
+      DIExpression::prependOpcodes(DVI->getExpression(), Ops, true);
+  LLVM_DEBUG(dbgs() << "scev-salvage: Updating: " << *DVIRec.DVI << '\n');
+  DVI->setExpression(OldExpr);
+  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');
+  return true;
+}
+
 static bool
 DbgRewriteSalvageableDVIs(llvm::Loop *L, ScalarEvolution &SE,
                           llvm::PHINode *LSRInductionVar,
@@ -6178,6 +6197,7 @@
     if (!IVAddRec->isAffine())
       return false;
 
+    // The iteratioun count is required to recover location values.
     SCEVDbgValueBuilder IterCountExpr;
     IterCountExpr.pushValue(LSRInductionVar);
     if (!IterCountExpr.SCEVToIterCountExpr(*IVAddRec, SE))
@@ -6201,13 +6221,28 @@
         llvm::Type *Ty = DVIRec.DVI->getVariableLocationOp(0)->getType();
         DVIRec.DVI->setRawLocation(
             llvm::ValueAsMetadata::get(UndefValue::get(Ty)));
-        DVIRec.DVI->setExpression(DVIRec.Expr);
       }
-
-      Changed |= RewriteDVIUsingIterCount(DVIRec, IterCountExpr, SE);
+      // LSR may have updated the expression, so restore to the expression
+      // that corresponds to te cached SCEV.
+      DVIRec.DVI->setExpression(DVIRec.Expr);
+
+      LLVM_DEBUG(dbgs() << "scev-salvage: value to recover SCEV: "
+                        << *DVIRec.SCEV);
+
+      // Create a simple expression if the IV and value to salvage SCEVs
+      // start values differ by only a constant value.
+      if (Optional<APInt> Offset =
+              SE.computeConstantDifference(DVIRec.SCEV, SCEVInductionVar)) {
+        if (Offset.getValue().getMinSignedBits() <= 64)
+          Changed |= RewriteDVIUsingOffset(DVIRec, *LSRInductionVar,
+                                           Offset.getValue().getSExtValue());
+      } else {
+        Changed |= RewriteDVIUsingIterCount(DVIRec, IterCountExpr, SE);
+      }
     }
+
+    return Changed;
   }
-  return Changed;
 }
 
 /// Identify and cache salvageable DVI locations and expressions along with the


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