[GlobalISel][IRTranslator] Don't add debug info to constants emitted into the entry block

[aemerson]

Summary

Constants, including G_GLOBAL_VALUE, are all emitted into the entry block which lets us use the vreg def assuming it dominates all other users. However, it can cause jumpy debug behaviour since the DebugLoc attached to these MIs are from
a user instruction that could be in a different block.

This makes GlobalISel consistent with SelectionDAG and fixes PR40887.

[aprantl]

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

1722

If you are inserting an instruction into the entry block without a DebugLoc and there is no instruction with a DebugLoc before it, it will get added to the function prologue. This may not matter, but I just wanted to make sure that you are aware of that.

[aemerson]

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

1722

Thanks for the heads up. That seems reasonable to me.

[dsanders]

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

143

Should we limit this exception to G_CONSTANT and similar? Normal code can potentially be in the entry block too can't it?

1722

Does having a DebugLoc with line 0 have the same issue?

[aprantl]

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

1722

No, line 0 would force the prologue end like any other valid location.

[aemerson]

Use a debug loc with line 0 instead, fix a test that should have been using fastisel.

[dsanders]

LGTM

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

public:
   void createdInstr(MachineInstr &MI) override {
     assert(getCurrentInst() && "Inserted instruction without a current MI");
 
     // Only print the check message if we're actually checking it.
 #ifndef NDEBUG
     LLVM_DEBUG(dbgs() << "Checking DILocation from " << *CurrInst
                       << " was copied to " << MI);
 #endif
-    assert(CurrInst->getDebugLoc() == MI.getDebugLoc() &&
+    // We allow insts in the entry block to not have a debug loc because they
+    // could have originated from constants, and we don't want a jumpy debug
+    // experience.
+    assert(CurrInst->getDebugLoc() == MI.getDebugLoc() ||
+           MI.getParent() == &MI.getParent()->getParent()->front() &&
                "Line info was not transferred to all instructions");
   }
 };
 } // namespace
 #endif // ifndef NDEBUG
 
 
 void IRTranslator::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<StackProtector>();
bool IRTranslator::valueIsSplit(const Value &V,
   if (Offsets && !Offsets->empty())
     Offsets->clear();
   computeValueLLTs(*DL, *V.getType(), SplitTys, Offsets);
   return SplitTys.size() > 1;
 }
 
 bool IRTranslator::translate(const Instruction &Inst) {
   CurBuilder->setDebugLoc(Inst.getDebugLoc());
-  EntryBuilder->setDebugLoc(Inst.getDebugLoc());
+  // We only emit constants into the entry block from here. To prevent jumpy
+  // debug behaviour.
+  EntryBuilder->setDebugLoc(DebugLoc());
   switch(Inst.getOpcode()) {
 #define HANDLE_INST(NUM, OPCODE, CLASS)                                        \
   case Instruction::OPCODE:                                                    \
     return translate##OPCODE(Inst, *CurBuilder.get());
 #include "llvm/IR/Instruction.def"
   default:
     return false;
   }

llvm/test/CodeGen/AArch64/GlobalISel/constant-dbg-loc.ll

+; RUN: llc -mtriple aarch64 -O0 -stop-after=irtranslator -global-isel -verify-machineinstrs %s -o - 2>&1 | FileCheck %s
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "arm64-apple-ios5.0.0"
+
+@var1 = common global i32 0, align 4, !dbg !0
+@var2 = common global i32 0, align 4, !dbg !6
+
+; We check here that the G_GLOBAL_VALUE doesn't have a debug loc operand.
+define i32 @main() #0 !dbg !14 {
+  ; CHECK-LABEL: name: main
+  ; CHECK: bb.1.entry:
+  ; CHECK-NOT: debug-location !21
+  ; CHECK:   successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  ; CHECK:   [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+  ; CHECK:   [[GV:%[0-9]+]]:_(p0) = G_GLOBAL_VALUE @var1
+  ; CHECK:   [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+  ; CHECK:   [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 2
+  ; CHECK:   [[GV1:%[0-9]+]]:_(p0) = G_GLOBAL_VALUE @var2
+  ; CHECK:   [[FRAME_INDEX:%[0-9]+]]:_(p0) = G_FRAME_INDEX %stack.0.retval
+  ; CHECK:   G_STORE [[C]](s32), [[FRAME_INDEX]](p0) :: (store 4 into %ir.retval)
+  ; CHECK:   [[LOAD:%[0-9]+]]:_(s32) = G_LOAD [[GV]](p0), debug-location !17 :: (load 4 from @var1)
+  ; CHECK:   [[ICMP:%[0-9]+]]:_(s1) = G_ICMP intpred(eq), [[LOAD]](s32), [[C1]], debug-location !19
+  ; CHECK:   G_BRCOND [[ICMP]](s1), %bb.2, debug-location !20
+  ; CHECK:   G_BR %bb.3, debug-location !20
+  ; CHECK: bb.2.if.then:
+  ; CHECK:   successors: %bb.3(0x80000000)
+  ; CHECK:   G_STORE [[C2]](s32), [[GV1]](p0), debug-location !21 :: (store 4 into @var2)
+  ; CHECK: bb.3.if.end:
+  ; CHECK:   $w0 = COPY [[C]](s32), debug-location !24
+  ; CHECK:   RET_ReallyLR implicit $w0, debug-location !24
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  %0 = load i32, i32* @var1, align 4, !dbg !17
+  %cmp = icmp eq i32 %0, 1, !dbg !19
+  br i1 %cmp, label %if.then, label %if.end, !dbg !20
+
+if.then:
+  store i32 2, i32* @var2, align 4, !dbg !21
+  br label %if.end, !dbg !23
+
+if.end:
+  ret i32 0, !dbg !24
+}
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10, !11, !12}
+!llvm.ident = !{!13}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "var1", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: GNU)
+!3 = !DIFile(filename: "dbg.c", directory: "/")
+!4 = !{}
+!5 = !{!0, !6}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "var2", scope: !2, file: !3, line: 2, type: !8, isLocal: false, isDefinition: true)
+!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!9 = !{i32 2, !"Dwarf Version", i32 2}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"wchar_size", i32 4}
+!12 = !{i32 7, !"PIC Level", i32 2}
+!13 = !{!"clang"}
+!14 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 3, type: !15, scopeLine: 3, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !4)
+!15 = !DISubroutineType(types: !16)
+!16 = !{!8}
+!17 = !DILocation(line: 4, column: 7, scope: !18)
+!18 = distinct !DILexicalBlock(scope: !14, file: !3, line: 4, column: 7)
+!19 = !DILocation(line: 4, column: 12, scope: !18)
+!20 = !DILocation(line: 4, column: 7, scope: !14)
+!21 = !DILocation(line: 5, column: 10, scope: !22)
+!22 = distinct !DILexicalBlock(scope: !18, file: !3, line: 4, column: 18)
+!23 = !DILocation(line: 6, column: 3, scope: !22)
+!24 = !DILocation(line: 7, column: 3, scope: !14)