[llvm] [DebugInfo] Preserve line and column number when merging debug info. (PR #129960)
Snehasish Kumar via llvm-commits
llvm-commits at lists.llvm.org
Thu Mar 6 14:22:04 PST 2025
https://github.com/snehasish updated https://github.com/llvm/llvm-project/pull/129960
>From cb23baa91558598ffff660bd68164a4d6ba0d3ea Mon Sep 17 00:00:00 2001
From: Snehasish Kumar <snehasishk at google.com>
Date: Wed, 5 Mar 2025 22:52:03 +0000
Subject: [PATCH 1/2] [DebugInfo] Preserve line and column number when merging
debug info.
This patch introduces a new option `-preserve-merged-debug-info` to
preserve an arbitrary version of debug information when DILocations are
merged. This is intended to be used in production environments from
which sample based profiles are derived such as AutoFDO and MemProf.
With this patch we have see a 0.2% improvement on an internal workload
at Google when generating AutoFDO profiles. It also significantly
improves the ability for MemProf by preserving debug info for merged
call instructions used in the contextual profile.
Co-authored-by: Krzysztof Pszeniczny <kpszeniczny at google.com>
---
llvm/lib/IR/DebugInfoMetadata.cpp | 15 +++++
.../DebugInfo/preserve-merged-debug-info.ll | 65 +++++++++++++++++++
2 files changed, 80 insertions(+)
create mode 100644 llvm/test/DebugInfo/preserve-merged-debug-info.ll
diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index f975d4ca33ad9..b18db09820068 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -21,6 +21,7 @@
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
+#include "llvm/Support/CommandLine.h"
#include <numeric>
#include <optional>
@@ -34,6 +35,12 @@ cl::opt<bool> EnableFSDiscriminator(
cl::desc("Enable adding flow sensitive discriminators"));
} // namespace llvm
+// When true, preserves line and column number by picking one of the merged
+// location info in a deterministic manner to assist sample based PGO.
+static cl::opt<bool> PreserveMergedDebugInfo(
+ "preserve-merged-debug-info", cl::init(false), cl::Hidden,
+ cl::desc("Preserve line and column number when merging locations."));
+
uint32_t DIType::getAlignInBits() const {
return (getTag() == dwarf::DW_TAG_LLVM_ptrauth_type ? 0 : SubclassData32);
}
@@ -125,6 +132,14 @@ DILocation *DILocation::getMergedLocation(DILocation *LocA, DILocation *LocB) {
if (LocA == LocB)
return LocA;
+ if (PreserveMergedDebugInfo) {
+ auto A = std::make_tuple(LocA->getLine(), LocA->getColumn(),
+ LocA->getDiscriminator());
+ auto B = std::make_tuple(LocB->getLine(), LocB->getColumn(),
+ LocB->getDiscriminator());
+ return A < B ? LocA : LocB;
+ }
+
LLVMContext &C = LocA->getContext();
using LocVec = SmallVector<const DILocation *>;
diff --git a/llvm/test/DebugInfo/preserve-merged-debug-info.ll b/llvm/test/DebugInfo/preserve-merged-debug-info.ll
new file mode 100644
index 0000000000000..d2b5af0d2d2c3
--- /dev/null
+++ b/llvm/test/DebugInfo/preserve-merged-debug-info.ll
@@ -0,0 +1,65 @@
+; RUN: opt %s -passes=simplifycfg -hoist-common-insts -preserve-merged-debug-info -S | FileCheck %s
+; CHECK: tail call i32 @bar{{.*!dbg !}}[[TAG:[0-9]+]]
+; CHECK: ![[TAG]] = !DILocation(line: 9, column: 16, scope: !9)
+
+; ModuleID = '../llvm/test/DebugInfo/Inputs/debug-info-merge-call.c'
+source_filename = "../llvm/test/DebugInfo/Inputs/debug-info-merge-call.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define dso_local i32 @test(i32 noundef %n) local_unnamed_addr #0 !dbg !9 {
+entry:
+ %call = tail call i32 @foo(i32 noundef %n) #2, !dbg !12
+ %cmp1 = icmp sgt i32 %n, 100, !dbg !13
+ br i1 %cmp1, label %if.then, label %if.else, !dbg !13
+
+if.then: ; preds = %entry
+ %call2 = tail call i32 @bar(i32 noundef %n) #2, !dbg !14
+ %add = add nsw i32 %call2, %call, !dbg !15
+ br label %if.end, !dbg !16
+
+if.else: ; preds = %entry
+ %call4 = tail call i32 @bar(i32 noundef %n) #2, !dbg !17
+ br label %if.end
+
+if.end: ; preds = %if.else, %if.then
+ %r.0 = phi i32 [ %add, %if.then ], [ %call4, %if.else ], !dbg !18
+ ret i32 %r.0, !dbg !19
+}
+
+declare !dbg !20 i32 @foo(i32 noundef) local_unnamed_addr #1
+
+declare !dbg !21 i32 @bar(i32 noundef) local_unnamed_addr #1
+
+attributes #0 = { nounwind uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+attributes #1 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+attributes #2 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 21.0.0git (git at github.com:snehasish/llvm-project.git 6ce41db6b0275d060d6e60f88b96a1657024345c)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "../llvm/test/DebugInfo/Inputs/debug-info-merge-call.c", directory: "/usr/local/google/home/snehasishk/working/llvm-project/build-assert", checksumkind: CSK_MD5, checksum: "ac1be6c40dad11691922d600f9d55c55")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 8, !"PIC Level", i32 2}
+!6 = !{i32 7, !"PIE Level", i32 2}
+!7 = !{i32 7, !"uwtable", i32 2}
+!8 = !{!"clang version 21.0.0git (git at github.com:snehasish/llvm-project.git 6ce41db6b0275d060d6e60f88b96a1657024345c)"}
+!9 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 5, type: !10, scopeLine: 5, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!10 = !DISubroutineType(types: !11)
+!11 = !{}
+!12 = !DILocation(line: 7, column: 13, scope: !9)
+!13 = !DILocation(line: 8, column: 8, scope: !9)
+!14 = !DILocation(line: 9, column: 16, scope: !9)
+!15 = !DILocation(line: 9, column: 14, scope: !9)
+!16 = !DILocation(line: 10, column: 3, scope: !9)
+!17 = !DILocation(line: 11, column: 10, scope: !9)
+!18 = !DILocation(line: 0, scope: !9)
+!19 = !DILocation(line: 13, column: 3, scope: !9)
+!20 = !DISubprogram(name: "foo", scope: !1, file: !1, line: 2, type: !10, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!21 = !DISubprogram(name: "bar", scope: !1, file: !1, line: 1, type: !10, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+
>From df04e25cffc85b3a4d9721aad68e0fab2f3f6453 Mon Sep 17 00:00:00 2001
From: Snehasish Kumar <snehasishk at google.com>
Date: Wed, 5 Mar 2025 22:52:03 +0000
Subject: [PATCH 2/2] Address comments.
---
.../DebugInfo/preserve-merged-debug-info.ll | 38 ++++++++++++-------
1 file changed, 25 insertions(+), 13 deletions(-)
diff --git a/llvm/test/DebugInfo/preserve-merged-debug-info.ll b/llvm/test/DebugInfo/preserve-merged-debug-info.ll
index d2b5af0d2d2c3..a5b7bf01c6020 100644
--- a/llvm/test/DebugInfo/preserve-merged-debug-info.ll
+++ b/llvm/test/DebugInfo/preserve-merged-debug-info.ll
@@ -1,6 +1,22 @@
-; RUN: opt %s -passes=simplifycfg -hoist-common-insts -preserve-merged-debug-info -S | FileCheck %s
-; CHECK: tail call i32 @bar{{.*!dbg !}}[[TAG:[0-9]+]]
-; CHECK: ![[TAG]] = !DILocation(line: 9, column: 16, scope: !9)
+;; This test verifies that we assign a deterministic location for merged
+;; instructions when -preserve-merged-debug-info is enabled. We use the
+;; simplifycfg pass to test this behaviour since it was a common source of
+;; merged instructions, however we intend this to apply to all users of the
+;; getMergedLocation API.
+
+;; Run simplifycfg and check that only 1 call to bar remains and it's debug
+;; location has a valid line number (lexicographically smallest).
+; RUN: opt %s -passes=simplifycfg -hoist-common-insts -preserve-merged-debug-info -S | FileCheck %s --check-prefix=ENABLED
+; ENABLED: call i32 @bar{{.*!dbg !}}[[TAG:[0-9]+]]
+; ENABLED-NOT: call i32 @bar
+; ENABLED: ![[TAG]] = !DILocation(line: 9, column: 16, scope: !9)
+
+;; Run simplifycfg without the pass to ensure that we don't spuriously start
+;; passing the test if simplifycfg behaviour changes.
+; RUN: opt %s -passes=simplifycfg -hoist-common-insts -preserve-merged-debug-info=false -S | FileCheck %s --check-prefix=DISABLED
+; DISABLED: call i32 @bar{{.*!dbg !}}[[TAG:[0-9]+]]
+; DISABLED-NOT: call i32 @bar
+; DISABLED: ![[TAG]] = !DILocation(line: 0, scope: !9)
; ModuleID = '../llvm/test/DebugInfo/Inputs/debug-info-merge-call.c'
source_filename = "../llvm/test/DebugInfo/Inputs/debug-info-merge-call.c"
@@ -8,19 +24,19 @@ target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:
target triple = "x86_64-unknown-linux-gnu"
; Function Attrs: nounwind uwtable
-define dso_local i32 @test(i32 noundef %n) local_unnamed_addr #0 !dbg !9 {
+define dso_local i32 @test(i32 %n) !dbg !9 {
entry:
- %call = tail call i32 @foo(i32 noundef %n) #2, !dbg !12
+ %call = call i32 @foo(i32 %n), !dbg !12
%cmp1 = icmp sgt i32 %n, 100, !dbg !13
br i1 %cmp1, label %if.then, label %if.else, !dbg !13
if.then: ; preds = %entry
- %call2 = tail call i32 @bar(i32 noundef %n) #2, !dbg !14
+ %call2 = call i32 @bar(i32 %n), !dbg !14
%add = add nsw i32 %call2, %call, !dbg !15
br label %if.end, !dbg !16
if.else: ; preds = %entry
- %call4 = tail call i32 @bar(i32 noundef %n) #2, !dbg !17
+ %call4 = call i32 @bar(i32 %n), !dbg !17
br label %if.end
if.end: ; preds = %if.else, %if.then
@@ -28,13 +44,9 @@ if.end: ; preds = %if.else, %if.then
ret i32 %r.0, !dbg !19
}
-declare !dbg !20 i32 @foo(i32 noundef) local_unnamed_addr #1
-
-declare !dbg !21 i32 @bar(i32 noundef) local_unnamed_addr #1
+declare !dbg !20 i32 @foo(i32)
-attributes #0 = { nounwind uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
-attributes #1 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
-attributes #2 = { nounwind }
+declare !dbg !21 i32 @bar(i32)
!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!2, !3, !4, !5, !6, !7}
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