[DebugInfo][InstrRef] Accept landingpad block arguments

[jmorse]

Summary

This patch makes instruction-referencing accepts an additional scenario where values can be read from physical registers at the start of blocks. As far as I was aware, this only happened:

• With arguments in the entry block,
• With constant physical registers,

To which this patch adds a third case:

• With exception-handling landing-pad blocks.

Observe the llvm-ir in the attached test: the operand of the dbg.value traces back to the "landingpad" instruction, which becomes some copies from physregs. Right now, that's deemed unacceptable, and the assertion fires.

The fix is to just accept this scenario; this is a case where the value in question is defined by a register and a position, not by an instruction that defines it. Reading it with a DBG_PHI is the correct behaviour, there isn't a non-copy instruction that we can refer to.

[Orlando]

LGTM. Please appease clang-format bot before landing.

llvm/test/DebugInfo/X86/instr-ref-eh-landing-pad.ll

31–32

Is that an accidental newline in there?

[jmorse]

llvm/test/DebugInfo/X86/instr-ref-eh-landing-pad.ll

31–32

This is how the IR printer chooses to render it -- not completely sure why, but it's canonical at the least.

llvm/lib/CodeGen/MachineFunction.cpp

if (TRI.isConstantPhysReg(State.first)) {
     // LLVM IR is allowed to read the framepointer by calling a
     // llvm.frameaddress.* intrinsic. We can support this by emitting a
     // DBG_PHI $fp. This isn't ideal, because it extends the behaviours /
     // position that DBG_PHIs appear at, limiting what can be done later.
     // TODO: see if there's a better way of expressing these variable
     // locations.
     ;
   } else {
-    // Assert that this is the entry block. If it isn't, then there is some
-    // code construct we don't recognise that deals with physregs across
-    // blocks.
+    // Assert that this is the entry block, or an EH pad. If it isn't, then
+    // there is some code construct we don't recognise that deals with physregs
+    // across blocks.
     assert(!State.first.isVirtual());
-    assert(&*InsertBB.getParent()->begin() == &InsertBB);
+    assert(&*InsertBB.getParent()->begin() == &InsertBB ||
+           InsertBB.isEHPad());
   }
 
   // Create DBG_PHI for specified physreg.
   auto Builder = BuildMI(InsertBB, InsertBB.getFirstNonPHI(), DebugLoc(),
                          TII.get(TargetOpcode::DBG_PHI));
   Builder.addReg(State.first, RegState::Debug);
   unsigned NewNum = getNewDebugInstrNum();
   Builder.addImm(NewNum);

llvm/test/DebugInfo/X86/instr-ref-eh-landing-pad.ll

+; RUN: llc %s -stop-before=finalize-isel -o -\
+; RUN:     -experimental-debug-variable-locations \
+; RUN:   | FileCheck %s
+;
+; Test that instruction referencing variable locations can cope with exception
+; landing pads. Variable locations can be derived from the ABI-defined arguments
+; to landing-pad blocks, which should be treated much like argument locations.
+; This gloriously simple piece of C++:
+;
+;    void a() try { a(); } catch (int *&) { }
+;
+; Produces the code below. A nameless variable is attached to the landing-pad
+; record %0 (via %3), which salvages back to the entry to the landing pad. We
+; should place a DBG_PHI at that point. Instead of crashing.
+;
+; CHECK-LABEL: bb.1.lpad (landing-pad):
+; CHECK:       DBG_PHI $rax, 1
+; CHECK-NEXT:  EH_LABEL
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@_ZTIPi = external dso_local constant i8*
+
+define dso_local void @_Z1av() local_unnamed_addr #0 personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) !dbg !8 {
+entry:
+  invoke void @_Z1av()
+          to label %try.cont unwind label %lpad, !dbg !17
+
+lpad:                                             ; preds = %entry
+  %0 = landingpad { i8*, i32 }
+          catch i8* bitcast (i8** @_ZTIPi to i8*), !dbg !19
+  %1 = extractvalue { i8*, i32 } %0, 1, !dbg !19
+  %2 = tail call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIPi to i8*)) #3, !dbg !20
+  %matches = icmp eq i32 %1, %2, !dbg !20
+  br i1 %matches, label %catch, label %eh.resume, !dbg !20
+
+catch:                                            ; preds = %lpad
+  %3 = extractvalue { i8*, i32 } %0, 0, !dbg !19
+  %4 = tail call i8* @__cxa_begin_catch(i8* %3) #3, !dbg !20
+  call void @llvm.dbg.value(metadata i8* %3, metadata !13, metadata !DIExpression(DW_OP_plus_uconst, 32, DW_OP_stack_value)), !dbg !21
+  tail call void @__cxa_end_catch() #3, !dbg !22
+  br label %try.cont, !dbg !22
+
+try.cont:                                         ; preds = %entry, %catch
+  ret void, !dbg !24
+
+eh.resume:                                        ; preds = %lpad
+  resume { i8*, i32 } %0, !dbg !20
+}
+
+declare dso_local i32 @__gxx_personality_v0(...)
+
+; Function Attrs: nofree nosync nounwind readnone
+declare i32 @llvm.eh.typeid.for(i8*) #1
+
+declare dso_local i8* @__cxa_begin_catch(i8*) local_unnamed_addr
+
+declare dso_local void @__cxa_end_catch() local_unnamed_addr
+
+; Function Attrs: mustprogress nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: ".")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{i32 7, !"uwtable", i32 1}
+!7 = !{!"clang"}
+!8 = distinct !DISubprogram(name: "a", linkageName: "_Z1av", scope: !9, file: !9, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!9 = !DIFile(filename: "test.cpp", directory: "")
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !{!13}
+!13 = !DILocalVariable(scope: !8, file: !9, line: 1, type: !14)
+!14 = !DIDerivedType(tag: DW_TAG_reference_type, baseType: !15, size: 64)
+!15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !DILocation(line: 1, column: 16, scope: !18)
+!18 = distinct !DILexicalBlock(scope: !8, file: !9, line: 1, column: 14)
+!19 = !DILocation(line: 2, column: 1, scope: !18)
+!20 = !DILocation(line: 1, column: 21, scope: !18)
+!21 = !DILocation(line: 0, scope: !8)
+!22 = !DILocation(line: 2, column: 1, scope: !23)
+!23 = distinct !DILexicalBlock(scope: !8, file: !9, line: 1, column: 38)
+!24 = !DILocation(line: 2, column: 1, scope: !8)