[PATCH] D42566: [InstCombine] Preserve debug values for eliminable casts

[Vedant Kumar via Phabricator via llvm-commits]

vsk created this revision.
vsk added reviewers: aprantl, rnk, dblaikie.

A cast from A to B is eliminable if its result is casted to C, and if
the pair of casts could just be expressed as a single cast. E.g here,
%c1 is eliminable:

  %c1 = zext i16 %A to i32
  %c2 = sext i32 %c1 to i64

InstCombine optimizes away eliminable casts. This patch teaches it to
insert a dbg.value intrinsic pointing to the final result, so that local
variables pointing to the eliminable result are preserved.


https://reviews.llvm.org/D42566

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/alloca-cast-debuginfo.ll
  test/Transforms/InstCombine/debuginfo-variables.ll


Index: test/Transforms/InstCombine/debuginfo-variables.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/debuginfo-variables.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -debugify -instcombine -S | FileCheck %s
+
+define i64 @test_sext_zext(i16 %A) {
+; CHECK-LABEL: @test_sext_zext(
+; CHECK-NEXT:  [[C2:%.*]] = zext i16 %A to i64
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i64 [[C2]], metadata !8, metadata !DIExpression()), !dbg !13
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i64 [[C2]], metadata !10, metadata !DIExpression()), !dbg !12
+  %c1 = zext i16 %A to i32
+  %c2 = sext i32 %c1 to i64
+  ret i64 %c2
+}
+
+; CHECK: !8 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !9)
+; CHECK: !9 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+; CHECK: !10 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 2, type: !11)
+; CHECK: !11 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+; CHECK: !12 = !DILocation(line: 2, column: 1, scope: !5)
+; CHECK: !13 = !DILocation(line: 1, column: 1, scope: !5)
Index: test/Transforms/InstCombine/alloca-cast-debuginfo.ll
===================================================================
--- test/Transforms/InstCombine/alloca-cast-debuginfo.ll
+++ test/Transforms/InstCombine/alloca-cast-debuginfo.ll
@@ -40,6 +40,10 @@
 ; CHECK-LABEL: define void @f(%struct.Foo* %p)
 ; CHECK: %local = alloca i64, align 8
 ; CHECK: call void @llvm.dbg.declare(metadata i64* %local, metadata !22, metadata !DIExpression())
+; CHECK: [[simplified:%.*]] = bitcast i64* %local to i8*
+; CHECK: call void @llvm.dbg.value(metadata i8* [[simplified]], metadata !22, metadata !DIExpression())
+; CHECK: call void @escape(i8* [[simplified]])
+; CHECK: ret void
 
 declare void @llvm.dbg.declare(metadata, metadata, metadata)
 
Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/KnownBits.h"
 using namespace llvm;
@@ -265,7 +266,20 @@
     if (Instruction::CastOps NewOpc = isEliminableCastPair(CSrc, &CI)) {
       // The first cast (CSrc) is eliminable so we need to fix up or replace
       // the second cast (CI). CSrc will then have a good chance of being dead.
-      return CastInst::Create(NewOpc, CSrc->getOperand(0), CI.getType());
+      auto *Res = CastInst::Create(NewOpc, CSrc->getOperand(0), CI.getType());
+
+      // If the eliminable cast has debug users, insert a debug value after the
+      // cast pointing to the new Value.
+      SmallVector<DbgInfoIntrinsic *, 1> CSrcDbgInsts;
+      findDbgUsers(CSrcDbgInsts, CSrc);
+      if (CSrcDbgInsts.size()) {
+        DIBuilder DIB(*CI.getModule());
+        for (auto *DII : CSrcDbgInsts)
+          DIB.insertDbgValueIntrinsic(
+              Res, DII->getVariable(), DII->getExpression(),
+              DII->getDebugLoc().get(), &*std::next(CI.getIterator()));
+      }
+      return Res;
     }
   }
 


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