[DebugInfo][InstrRef] Correctly ignore DBG_VALUE_LIST in InstrRef mode, for now

[jmorse]

Summary

This patch makes InstrRefBasedLDV "safe" to work with DBG_VALUE_LISTs. It doesn't actually interpret them, but it recognises that they specify variable locations and avoids propagating false locations, which is better than the current state. Observe the attached test:

• We avoid propagating DBG_VALUE_LISTs into successor blocks, as they're not "currently" supported,
• We don't propagate other variable locations across DBG_VALUE_LISTs, because we know that the variable location is terminated by the DBG_VALUE_LIST.

I'd like to get this patch into the llvm-13 branch, so that anyone experimenting with this in the meantime gets safe variable locations. Then we can get on to properly supporting DBG_VALUE_LISTs.

[Orlando]

LGTM

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

bool InstrRefBasedLDV::transferDebugValue(const MachineInstr &MI) {
   DbgValueProperties Properties(MI);
 
   // If there are no instructions in this lexical scope, do no location tracking
   // at all, this variable shouldn't get a legitimate location range.
   auto *Scope = LS.findLexicalScope(MI.getDebugLoc().get());
   if (Scope == nullptr)
     return true; // handled it; by doing nothing
 
+  // For now, ignore DBG_VALUE_LISTs when extending ranges. Allow it to
+  // contribute to locations in this block, but don't propagate further.
+  // Interpret it like a DBG_VALUE $noreg.
+  if (MI.isDebugValueList()) {
+    if (VTracker)
+      VTracker->defVar(MI, Properties, None);
+    if (TTracker)
+      TTracker->redefVar(MI, Properties, None);
+    return true;
+  }
+
   const MachineOperand &MO = MI.getOperand(0);
 
   // MLocTracker needs to know that this register is read, even if it's only
   // read by a debug inst.
   if (MO.isReg() && MO.getReg() != 0)
     (void)MTracker->readReg(MO.getReg());
 
   // If we're preparing for the second analysis (variables), the machine value

llvm/test/DebugInfo/MIR/InstrRef/ignore-dbg-value-list.mir

+# RUN: llc %s -o - -mtriple=x86_64-unknown-unknown \
+# RUN:    -experimental-debug-variable-locations -run-pass=livedebugvalues\
+# RUN:    | FileCheck %s --implicit-check-not=DBG_VALUE \
+# RUN:                   --implicit-check-not=DBG_VALUE_LIST
+#
+# Test that any DBG_VALUE_LISTs observed are interpreted as DBG_VALUE $noreg.
+# This is obviously sub-optimal, but avoids false variable locations, for the
+# period of time until InstrRefBasedLDV supports these variable locations.
+# 
+--- |
+  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"
+  
+  define dso_local i32 @foo() !dbg !7 {
+  entry:
+    ret i32 0
+  }
+  
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4, !5}
+  !llvm.ident = !{!6}
+  
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+  !1 = !DIFile(filename: "test.c", directory: "/tmp/out.c")
+  !2 = !{}
+  !3 = !{i32 7, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{i32 1, !"wchar_size", i32 4}
+  !6 = !{!""}
+  !7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!10, !11, !11}
+  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !11 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+  !12 = !DILocalVariable(name: "bar", arg: 1, scope: !7, file: !1, line: 3, type: !11)
+  !13 = !DILocation(line: 0, scope: !7)
+  !14 = !DILocalVariable(name: "baz", arg: 2, scope: !7, file: !1, line: 3, type: !11)
+
+...
+---
+name:            foo
+alignment:       16
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $rdi, $rsi, $r14, $rbx
+
+    ; CHECK-LABEL: bb.0:
+    $rax = MOV64ri 0, debug-location !13
+    $rbx = MOV64ri 0, debug-location !13
+    DBG_VALUE_LIST !12, !DIExpression(), $rax, $rbx, debug-location !13
+    ; CHECK: DBG_VALUE_LIST
+
+  bb.1:
+    liveins: $rax
+
+    ; CHECK-LABEL: bb.1:
+    ; Earlier DBG_VALUE_LIST should not be propagated here, would be caught
+    ; by implicit check not on FileCheck cmdline.
+
+    DBG_VALUE $rax, $noreg, !12, !DIExpression(), debug-location !13
+    ; CHECK: DBG_VALUE
+    $rbx = MOV64ri 0, debug-location !13
+    DBG_VALUE_LIST !12, !DIExpression(), $rax, $rbx, debug-location !13
+    ; CHECK: DBG_VALUE_LIST
+    $rbx = COPY killed $rax, debug-location !13
+    $rax = MOV64ri 0, debug-location !13
+    ; This clobber of $rax might cause LDV to re-issue a DBG_VALUE stating the
+    ; variable location as $rbx. However, the preceeding DBG_VALUE_LIST should
+    ; terminate the earlier location.
+    RETQ implicit $rbx, debug-location !13
+
+...