{DebugInfo][LoopStrengthReduction] Don't cache dbg.value that are already undef

[chrisjackson]

Summary

The SCEV-based salvaging method caches dbg.value information pre-LSR so that salvaging may be attempted on those dbg.value if necessary post-LSR. If the dbg.value are already undef pre-LSR then a salvage attempt would be fruitless, so avoid caching them.

[chrisjackson]

Applied clang-format.

[StephenTozer]

Sensible; unfortunately, it probably needs a test - including an extra undef dbg.value in the test on the previous patch will probably do.

[chrisjackson]

In D107448#2925184, @StephenTozer wrote:

Sensible; unfortunately, it probably needs a test - including an extra undef dbg.value in the test on the previous patch will probably do.

I will add an undef dbg.value as you suggest as a form of regression test.

[chrisjackson]

Add updated test that is shared with D107438.

[chrisjackson]

Correct test file format.

[chrisjackson]

Update patch to apply on top of https://github.com/llvm/llvm-project/commit/914066fe38a93c004b742a696ec337701eb738ec

[StephenTozer]

LGTM.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

DbgGatherSalvagableDVI(Loop *L, ScalarEvolution &SE,
                        SmallVector<DVIRecoveryRec, 2> &SalvageableDVISCEVs,
                        SmallSet<AssertingVH<DbgValueInst>, 2> &DVIHandles) {
   for (auto &B : L->getBlocks()) {
     for (auto &I : *B) {
       auto DVI = dyn_cast<DbgValueInst>(&I);
       if (!DVI)
         continue;
 
+      if (DVI->isUndef())
+        continue;
+
       if (DVI->hasArgList())
         continue;
 
       if (!DVI->getVariableLocationOp(0) ||
           !SE.isSCEVable(DVI->getVariableLocationOp(0)->getType()))
         continue;
 
       SalvageableDVISCEVs.push_back(

llvm/test/Transforms/LoopStrengthReduce/pr51329.ll

 ; RUN: opt -S -loop-reduce %s | FileCheck %s
 ;
 ; Test that LSR SCEV-based salvaging does not crash when translating SCEVs
 ; that contain integers with binary representations greater than 64-bits. 
+; Also show that no salvaging attempt is made for dbg.value that are undef
+; pre-LSR.
 ;
+; CHECK: call void @llvm.dbg.value(metadata i64 undef, metadata !{{[0-9]+}}, metadata !DIExpression(DW_OP_plus_uconst, 228, DW_OP_stack_value))
 ; CHECK: call void @llvm.dbg.value(metadata i64 %var2, metadata !{{[0-9]+}}, metadata !DIExpression(DW_OP_plus_uconst, 228, DW_OP_stack_value))
 
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
 declare void @llvm.dbg.value(metadata, metadata, metadata)
 
 ; Function Attrs: nounwind
 define hidden void @reproducer() local_unnamed_addr !dbg !5 {
 init:
   %0 = lshr i128 undef, 64
   %var1 = trunc i128 %0 to i64
   %1 = add nuw i64 undef, %var1
   %var2 = lshr i64 %1, 12
   br label %Label_d0
 
 Label_d0:                                         ; preds = %Label_d0, %init
   %var3 = phi i64 [ %var2, %init ], [ %var4, %Label_d0 ]
+  call void @llvm.dbg.value(metadata i64 undef, metadata !11, metadata !DIExpression(DW_OP_plus_uconst, 228, DW_OP_stack_value)), !dbg !12
   call void @llvm.dbg.value(metadata i64 %var2, metadata !11, metadata !DIExpression(DW_OP_plus_uconst, 228, DW_OP_stack_value)), !dbg !12
   %var4 = add i64 %var3, -1
   %var5 = icmp eq i64 %var4, 0
   br i1 %var5, label %Label_1bc, label %Label_d0
 
 Label_1bc:                                        ; preds = %Label_d0
   ret void
 }