[DebugInfo] Don't set prologue_end behind line-zero call insts

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -2211,13 +2211,19 @@ void DwarfDebug::beginInstruction(const MachineInstr *MI) {
     PrevInstLoc = DL;
 }
 
+// Returns the position where we should place prologue_end, potentially nullptr,
+// which means "no good place to put prologue_end". Returns true in the second
+// return value if there are no setup instructions in this function at all,
+// meaning we should not emit a start-of-function linetable entry, because it
+// would be zero-lengthed.
 static std::pair<const MachineInstr *, bool>
 findPrologueEndLoc(const MachineFunction *MF) {
   // First known non-DBG_VALUE and non-frame setup location marks
   // the beginning of the function body.
   const auto &TII = *MF->getSubtarget().getInstrInfo();
   const MachineInstr *NonTrivialInst = nullptr;
   const Function &F = MF->getFunction();
+  DISubprogram *SP = const_cast<DISubprogram *>(F.getSubprogram());
 
   // Some instructions may be inserted into prologue after this function. Must
   // keep prologue for these cases.
@@ -2305,6 +2311,26 @@ findPrologueEndLoc(const MachineFunction *MF) {
         return *FoundInst;
     }
 
+    // In very rare scenarios function calls can have line zero, and we
+    // shouldn't step over such a call while trying to reach prologue_end. In
+    // these extraordinary conditions, force the call to have the scope line
+    // and put prologue_end there. This isn't ideal, but signals that the call
+    // is where execution in the function starts, and is less catastrophic than
+    // stepping over the call.
+    if (CurInst->isCall()) {
+      if (const DILocation *Loc = CurInst->getDebugLoc().get();
+          Loc && Loc->getLine() == 0) {
+        // Create and assign the scope-line position.
+        unsigned ScopeLine = SP->getScopeLine();
+        DILocation *ScopeLineDILoc =
+            DILocation::get(SP->getContext(), ScopeLine, 0, SP);
+        const_cast<MachineInstr *>(&*CurInst)->setDebugLoc(ScopeLineDILoc);
+
+        // Consider this position to be where prologue_end is placed.
+        return std::make_pair(&*CurInst, false);
+      }
+    }
+
     // Try to continue searching, but use a backup-location if substantive
     // computation is happening.
     auto NextInst = std::next(CurInst);

llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir

@@ -5,7 +5,7 @@
 # CHECK: Ltmp0:
 # CHECK: .loc 1 0 0
 # CHECK-NOT: .loc 1 0 0
-# CHECK: .loc 1 37 1 prologue_end
+# CHECK: .loc 1 37 1
 
 --- |
   ; ModuleID = '<stdin>'

llvm/test/DebugInfo/X86/no-prologue-end-after-line0-calls.mir

@@ -0,0 +1,132 @@
+# RUN: llc %s -start-after=livedebugvalues -o - | FileCheck %s
+#
+## Original code, compiled clang -O2 -g -c
+## 
+## void ext();
+## int main(int argc, char **argv) {
+##   if (argc == 1)
+##     ext();
+##   else
+##     ext();
+##    return 0;
+## }
+## 
+## In the code sequence above, the call to ext is given line zero during
+## optimisation, because the code is duplicated down all function paths thus
+## gets merged. We get something like this as the output:
+##
+##   0:   50                      push   %rax
+##   1:   31 c0                   xor    %eax,%eax
+##   3:   e8 00 00 00 00          call   8 <main+0x8>
+##   4: R_X86_64_PLT32       ext-0x4
+##   8:   31 c0                   xor    %eax,%eax
+##   a:   59                      pop    %rcx
+##   b:   c3                      ret
+##
+## And we could choose to set prologue_end on address 8, the clearing of the
+## return register, because it's the first "real" instruction that isn't line
+## zero. But this then causes debuggers to skip over the call instruction when
+## entering the function, which is catastrophic.
+##
+## Instead: force the call itself to have a source location (the function scope
+## line number), and put a prologue_end there. While it's not the original
+## source of the call, it's better to have a prologue_end that means we'll stop
+## in the prologue than to step over the call. This gives consumers the
+## opportunity to recognise "this is a crazy function" and act accordingly.
+##
+## Check lines: ensure that we set prologue_end. The first entry is the
+## start-of-function scope line, the second entry is the prologue_end on the
+## call.
+#
+#
+# CHECK:         main:
+# CHECK-NEXT:    .Lfunc_begin0:
+# CHECK-NEXT:        .file   0 "/tmp/test.c"
+# CHECK-NEXT:        .loc    0 2 0
+# CHECK-NEXT:        .cfi_startproc
+# CHECK-NEXT:    # %bb.0:
+# CHECK-NEXT:        pushq   %rax
+# CHECK-NEXT:        .cfi_def_cfa_offset 16
+# CHECK-NEXT:    .Ltmp0:
+# CHECK-NEXT:        .loc    0 0 0 is_stmt 0
+# CHECK-NEXT:        xorl    %eax, %eax
+# CHECK-NEXT:        .loc    0 2 0 prologue_end is_stmt 1
+# CHECK-NEXT:        callq   ext@PLT
+
+--- |
+  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 noundef i32 @main(i32 noundef %argc, ptr noundef readnone captures(none) %argv) local_unnamed_addr !dbg !10 {
+  entry:
+    tail call void (...) @ext(), !dbg !22
+    ret i32 0, !dbg !24
+  }
+
+  declare !dbg !25 void @ext(...) local_unnamed_addr
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!2, !3, !4, !5, !6, !7, !8}
+  !llvm.ident = !{!9}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+  !1 = !DIFile(filename: "/tmp/test.c", directory: "")
+  !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 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
+  !9 = !{!"clang"}
+  !10 = distinct !DISubprogram(name: "main", scope: !11, file: !11, line: 2, type: !12, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !18, keyInstructions: true)
+  !11 = !DIFile(filename: "/tmp/test.c", directory: "")
+  !12 = !DISubroutineType(types: !13)
+  !13 = !{!14, !14, !15}
+  !14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
+  !16 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !17, size: 64)
+  !17 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+  !18 = !{!19, !20}
+  !19 = !DILocalVariable(name: "argc", arg: 1, scope: !10, file: !11, line: 2, type: !14)
+  !20 = !DILocalVariable(name: "argv", arg: 2, scope: !10, file: !11, line: 2, type: !15)
+  !21 = !DILocation(line: 0, scope: !10)
+  !22 = !DILocation(line: 0, scope: !23)
+  !23 = distinct !DILexicalBlock(scope: !10, file: !11, line: 3, column: 7)
+  !24 = !DILocation(line: 7, column: 4, scope: !10, atomGroup: 2, atomRank: 1)
+  !25 = !DISubprogram(name: "ext", scope: !11, file: !11, line: 1, type: !26, spFlags: DISPFlagOptimized)
+  !26 = !DISubroutineType(types: !27)
+  !27 = !{null}
+...
+---
+name:            main
+alignment:       16
+tracksRegLiveness: true
+noPhis:          true
+isSSA:           false
+noVRegs:         true
+hasFakeUses:     false
+debugInstrRef:   true
+tracksDebugUserValues: true
+frameInfo:
+  stackSize:       8
+  offsetAdjustment: -8
+  maxAlignment:    1
+  adjustsStack:    true
+  hasCalls:        true
+  maxCallFrameSize: 0
+  isCalleeSavedInfoValid: true
+machineFunctionInfo:
+  amxProgModel:    None
+body:             |
+  bb.0.entry:
+    frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 16
+    dead $eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags, implicit-def $al,  debug-location !22
+    CALL64pcrel32 target-flags(x86-plt) @ext, csr_64, implicit $rsp, implicit $ssp, implicit killed $al, implicit-def $rsp, implicit-def $ssp,  debug-location !22
+    $eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags,  debug-location !24
+    $rcx = frame-destroy POP64r implicit-def $rsp, implicit $rsp,  debug-location !24
+    frame-destroy CFI_INSTRUCTION def_cfa_offset 8,  debug-location !24
+    RET64 $eax,  debug-location !24
+...