[DebugInfo] Don't salvage load operations (PR40628).

Salvaging a redundant load instruction into a debug expression hides a
memory read from optimisation passes. Passes that alter memory behaviour
(such as LICM promoting memory to a register) aren't aware of these debug
memory reads and leave them unaltered, making the debug variable location
point somewhere unsafe.

Teaching passes to know about these debug memory reads would be challenging
and probably incomplete. Finding dbg.value instructions that need to be fixed
would likely be computationally expensive too, as more analysis would be
required. It's better to not generate debug-memory-reads instead, alas.

Changed tests:
 * DeadStoreElim: test for salvaging of intermediate operations contributing
   to the dead store, instead of salvaging of the redundant load,
 * GVN: remove debuginfo behaviour checks completely, this behaviour is still
   covered by other tests,
 * InstCombine: don't test for salvaged loads, we're removing that behaviour.

Differential Revision: https://reviews.llvm.org/D57962

llvm-svn: 353824

4 files changed, 66 insertions, 16 deletions

diff --git a/llvm/test/DebugInfo/Generic/pr40628.ll b/llvm/test/DebugInfo/Generic/pr40628.ll
new file mode 100644
index 00000000000..69f09f7eb5b
--- /dev/null
+++ b/llvm/test/DebugInfo/Generic/pr40628.ll
@@ -0,0 +1,56 @@
+; RUN: opt -early-cse -S %s -o - | FileCheck %s
+
+; PR40628: The first load below is determined to be redundant by EarlyCSE.
+; During salvaging, the corresponding dbg.value could have a DW_OP_deref used
+; in its DIExpression to represent the redundant operation -- however LLVM
+; cannot currently determine that the subsequent store should terminate the
+; variables location range. A debugger would display zero for the "redundant"
+; variable after stepping onto the return instruction.
+
+; Test that the load being removed results in the corresponding dbg.value
+; being assigned the 'undef' value.
+
+; CHECK:      @foo
+; CHECK-NEXT: dbg.value(metadata i32 undef, metadata ![[DEADVAR:[0-9]+]],
+; CHECK-NEXT: load
+; CHECK-NEXT: dbg.value(metadata i32 %{{[0-9]+}}, metadata ![[LIVEVAR:[0-9]+]],
+; CHECK-NEXT: store
+; CHECK-NEXT: ret
+
+; CHECK:      ![[DEADVAR]] = !DILocalVariable(name: "redundant",
+; CHECK:      ![[LIVEVAR]] = !DILocalVariable(name: "loaded",
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define dso_local i32 @foo(i32*) !dbg !7 {
+  %2 = load i32, i32* %0, align 4, !dbg !23
+  call void @llvm.dbg.value(metadata i32 %2, metadata !16, metadata !DIExpression()), !dbg !23
+  %3 = load i32, i32* %0, align 4, !dbg !23
+  call void @llvm.dbg.value(metadata i32 %3, metadata !17, metadata !DIExpression()), !dbg !23
+  store i32 0, i32* %0, align 4, !dbg !23
+  ret i32 %3, !dbg !23
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "pr40628.c", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 2, type: !8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !11}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !10, size: 64)
+!12 = !{!16, !17}
+!16 = !DILocalVariable(name: "redundant", scope: !7, file: !1, line: 4, type: !10)
+!17 = !DILocalVariable(name: "loaded", scope: !7, file: !1, line: 5, type: !10)
+!23 = !DILocation(line: 4, column: 7, scope: !7)
diff --git a/llvm/test/Transforms/DeadStoreElimination/debuginfo.ll b/llvm/test/Transforms/DeadStoreElimination/debuginfo.ll
index 2d9b4fe3ee8..b14c779288a 100644
--- a/llvm/test/Transforms/DeadStoreElimination/debuginfo.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/debuginfo.ll
@@ -6,23 +6,21 @@ declare noalias i8* @malloc(i32)
 
 declare void @test_f()
 
-define i32* @test_salvage() {
+define i32* @test_salvage(i32 %arg) {
 ; Check that all four original local variables have their values preserved.
-; CHECK-LABEL: @test_salvage()
+; CHECK-LABEL: @test_salvage(
 ; CHECK-NEXT: malloc
 ; CHECK-NEXT: @llvm.dbg.value(metadata i8* %p, metadata ![[p:.*]], metadata !DIExpression())
 ; CHECK-NEXT: bitcast
 ; CHECK-NEXT: @llvm.dbg.value(metadata i32* %P, metadata ![[P:.*]], metadata !DIExpression())
-; CHECK-NEXT: @llvm.dbg.value(metadata i32* %P, metadata ![[DEAD:.*]], metadata !DIExpression(DW_OP_deref))
-; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata ![[DEAD2:.*]], metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 1, DW_OP_stack_value))
+; CHECK-NEXT: @llvm.dbg.value(metadata i32 %arg, metadata ![[DEAD:.*]], metadata !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value))
 ; CHECK-NEXT: call void @test_f()
 ; CHECK-NEXT: store i32 0, i32* %P
 
   %p = tail call i8* @malloc(i32 4)
   %P = bitcast i8* %p to i32*
-  %DEAD = load i32, i32* %P
-  %DEAD2 = add i32 %DEAD, 1
-  store i32 %DEAD2, i32* %P
+  %DEAD = add i32 %arg, 1
+  store i32 %DEAD, i32* %P
   call void @test_f()
   store i32 0, i32* %P
   ret i32* %P
@@ -31,4 +29,3 @@ define i32* @test_salvage() {
 ; CHECK: ![[p]] = !DILocalVariable(name: "1"
 ; CHECK: ![[P]] = !DILocalVariable(name: "2"
 ; CHECK: ![[DEAD]] = !DILocalVariable(name: "3"
-; CHECK: ![[DEAD2]] = !DILocalVariable(name: "4"
diff --git a/llvm/test/Transforms/GVN/fence.ll b/llvm/test/Transforms/GVN/fence.ll
index ab8e2bbe72b..f39fb951865 100644
--- a/llvm/test/Transforms/GVN/fence.ll
+++ b/llvm/test/Transforms/GVN/fence.ll
@@ -1,4 +1,4 @@
-; RUN: opt -S -debugify -basicaa -gvn < %s | FileCheck %s
+; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
 
 @a = external constant i32
 ; We can value forward across the fence since we can (semantically) 
@@ -18,10 +18,8 @@ define i32 @test(i32* %addr.i) {
 ; Same as above
 define i32 @test2(i32* %addr.i) {
 ; CHECK-LABEL: @test2
-; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %addr.i, metadata [[var_a:![0-9]+]], metadata !DIExpression(DW_OP_deref))
 ; CHECK-NEXT: fence
 ; CHECK-NOT: load
-; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %addr.i, metadata [[var_a2:![0-9]+]], metadata !DIExpression(DW_OP_deref))
 ; CHECK: ret
   %a = load i32, i32* %addr.i, align 4
   fence release
@@ -88,6 +86,3 @@ define i32 @test4(i32* %addr) {
 ; }
 ; Given we chose to forward across the release fence, we clearly can't forward
 ; across the acquire fence as well.
-
-; CHECK: [[var_a]] = !DILocalVariable
-; CHECK-NEXT: [[var_a2]] = !DILocalVariable
diff --git a/llvm/test/Transforms/InstCombine/debuginfo-dce.ll b/llvm/test/Transforms/InstCombine/debuginfo-dce.ll
index d23cc1e9952..200ea26cdaf 100644
--- a/llvm/test/Transforms/InstCombine/debuginfo-dce.ll
+++ b/llvm/test/Transforms/InstCombine/debuginfo-dce.ll
@@ -23,6 +23,9 @@ target triple = "x86_64-apple-macosx10.12.0"
 
 %struct.entry = type { %struct.entry* }
 
+; This salvage can't currently occur safely (PR40628), however if/when that's
+; ever fixed, then this is definitely a piece of test coverage that should
+; be maintained.
 define void @salvage_load(%struct.entry** %queue) local_unnamed_addr #0 !dbg !14 {
 entry:
   %im_not_dead = alloca %struct.entry*
@@ -31,8 +34,7 @@ entry:
   call void @llvm.dbg.value(metadata %struct.entry* %1, metadata !18, metadata !20), !dbg !19
 ; CHECK: define void @salvage_load
 ; CHECK-NEXT: entry:
-; CHECK-NEXT: call void @llvm.dbg.value(metadata %struct.entry** %queue,
-; CHECK-SAME:                           metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 0))
+; CHECK-NOT: dbg.value
   store %struct.entry* %1, %struct.entry** %im_not_dead, align 8
   ret void, !dbg !21
 }