[llvm] [DebugInfo][DWARF] Set is_stmt on first non-line-0 instruction in BB (PR #105524)

[Stephen Tozer via llvm-commits]

https://github.com/SLTozer created https://github.com/llvm/llvm-project/pull/105524

Fixes: https://github.com/llvm/llvm-project/issues/104695

This patch adds the is_stmt flag to line table entries for the first instruction with a non-0 line location in each basic block, to ensure that it will be used for stepping even if the last instruction in the previous basic block had the same line number; this is important for cases where the new BB is reachable from BBs other than the preceding block.

>From cf8ef907bde71a8ac9f374a6590deaaa326b3359 Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Wed, 21 Aug 2024 14:12:08 +0100
Subject: [PATCH] [DebugInfo][DWARF] Set is_stmt on first non-line-0
 instruction in BB

Fixes: https://github.com/llvm/llvm-project/issues/104695

This patch adds the is_stmt flag to line table entries for the first
instruction with a non-0 line location in each basic block, to ensure
that it will be used for stepping even if the last instruction in the
previous basic block had the same line number; this is important for cases
where the new BB is reachable from BBs other than the preceding block.
---
 llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp    |  7 +++-
 .../Generic/is_stmt-at-block-start.ll         | 38 +++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/DebugInfo/Generic/is_stmt-at-block-start.ll

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index f111b4aea06f1b..aa11200731e1b3 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2119,9 +2119,12 @@ void DwarfDebug::beginInstruction(const MachineInstr *MI) {
     PrologEndLoc = DebugLoc();
   }
   // If the line changed, we call that a new statement; unless we went to
-  // line 0 and came back, in which case it is not a new statement.
+  // line 0 and came back, in which case it is not a new statement. We also
+  // mark is_stmt for the first non-0 line in each BB, in case a predecessor BB
+  // ends with a different line.
   unsigned OldLine = PrevInstLoc ? PrevInstLoc.getLine() : LastAsmLine;
-  if (DL.getLine() && DL.getLine() != OldLine)
+  if (DL.getLine() &&
+      (DL.getLine() != OldLine || PrevInstBB != MI->getParent()))
     Flags |= DWARF2_FLAG_IS_STMT;
 
   const MDNode *Scope = DL.getScope();
diff --git a/llvm/test/DebugInfo/Generic/is_stmt-at-block-start.ll b/llvm/test/DebugInfo/Generic/is_stmt-at-block-start.ll
new file mode 100644
index 00000000000000..21efceab38eca0
--- /dev/null
+++ b/llvm/test/DebugInfo/Generic/is_stmt-at-block-start.ll
@@ -0,0 +1,38 @@
+;; Checks that when an instruction at the start of a BasicBlock has the same
+;; DebugLoc as the instruction at the end of the previous BasicBlock, we add
+;; is_stmt to the new line, to ensure that we still step on it if we arrive from
+;; a BasicBlock other than the immediately preceding one.
+
+; RUN: %llc_dwarf -O0 -filetype=obj < %s | llvm-dwarfdump --debug-line - | FileCheck %s
+
+; CHECK:      {{0x[0-9a-f]+}}     13      5 {{.+}} is_stmt
+; CHECK-NEXT: {{0x[0-9a-f]+}}     13     25 {{.+}} is_stmt
+
+define void @_Z1fi(i1 %cond) !dbg !21 {
+entry:
+  br i1 %cond, label %if.then2, label %if.else4
+
+if.then2:                                         ; preds = %entry
+  br label %if.end8, !dbg !24
+
+if.else4:                                         ; preds = %entry
+  %0 = load i32, ptr null, align 4, !dbg !28
+  %call5 = call i1 null(i32 %0)
+  ret void
+
+if.end8:                                          ; preds = %if.then2
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!20}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 20.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/home/gbtozers/dev/upstream-llvm")
+!20 = !{i32 2, !"Debug Info Version", i32 3}
+!21 = distinct !DISubprogram(name: "f", linkageName: "_Z1fi", scope: !1, file: !1, line: 7, type: !22, scopeLine: 7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
+!22 = distinct !DISubroutineType(types: !23)
+!23 = !{null}
+!24 = !DILocation(line: 13, column: 5, scope: !25)
+!25 = distinct !DILexicalBlock(scope: !21, file: !1, line: 11, column: 27)
+!28 = !DILocation(line: 13, column: 25, scope: !25)