[DebugInfo][DWARF] Set is_stmt on first non-line-0 instruction in BB

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();

llvm/test/CodeGen/X86/fsafdo_test1.ll

@@ -4,9 +4,9 @@
 ; Check that fs-afdo discriminators are generated.
 ; V01: .loc    1 7 3 is_stmt 0 discriminator 2 # foo.c:7:3
 ; V01: .loc    1 9 5 is_stmt 1 discriminator 2 # foo.c:9:5
-; V0: .loc    1 9 5 discriminator 11266 # foo.c:9:5
+; V0: .loc    1 9 5 is_stmt 1 discriminator 11266 # foo.c:9:5
 ; V0: .loc    1 7 3 is_stmt 1 discriminator 11266 # foo.c:7:3
-; V1: .loc    1 9 5 discriminator 514 # foo.c:9:5
+; V1: .loc    1 9 5 is_stmt 1 discriminator 514 # foo.c:9:5
 ; V1: .loc    1 7 3 is_stmt 1 discriminator 258 # foo.c:7:3
 ; Check that variable __llvm_fs_discriminator__ is generated.
 ; V01: .type   __llvm_fs_discriminator__,@object # @__llvm_fs_discriminator__

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)

llvm/test/DebugInfo/X86/discriminator.ll

@@ -59,4 +59,4 @@ attributes #0 = { nounwind uwtable "less-precise-fpmad"="false" "frame-pointer"=
 
 ; CHECK: Address            Line   Column File   ISA Discriminator OpIndex Flags
 ; CHECK: ------------------ ------ ------ ------ --- ------------- ------- -------------
-; CHECK: 0x000000000000000a      2      0      1   0            42       0 {{$}}
+; CHECK: 0x000000000000000a      2      0      1   0            42       0 is_stmt{{$}}