[DebugInfo] Allow creation of DBG_VALUEs in blocks where the operand is not used

dbg.value intrinsics can appear in blocks where their operand is not used,
meaning the operand never receives an SDNode, and thus no DBG_VALUE will
be created. Get around this by looking to see whether the operand has already
been allocated a virtual register. This allows dbg.values of Phi node and
Values that are used across basic blocks to successfully be translated into
DBG_VALUEs.

Differential Revision: https://reviews.llvm.org/D56678

2 files changed, 66 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 871ab9b2988..e887af4ca58 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5344,14 +5344,15 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
       return nullptr;
     }
 
-    // PHI nodes have already been selected, so we should know which VReg that
-    // is assigns to already.
-    if (isa<PHINode>(V)) {
+    // The value is not used in this block yet (or it would have an SDNode).
+    // We still want the value to appear for the user if possible -- if it has
+    // an associated VReg, we can refer to that instead.
+    if (!isa<Argument>(V)) {
       auto VMI = FuncInfo.ValueMap.find(V);
       if (VMI != FuncInfo.ValueMap.end()) {
         unsigned Reg = VMI->second;
-        // The PHI node may be split up into several MI PHI nodes (in
-        // FunctionLoweringInfo::set).
+        // If this is a PHI node, it may be split up into several MI PHI nodes
+        // (in FunctionLoweringInfo::set).
         RegsForValue RFV(V->getContext(), TLI, DAG.getDataLayout(), Reg,
                          V->getType(), None);
         if (RFV.occupiesMultipleRegs()) {
diff --git a/llvm/test/DebugInfo/X86/sdag-dbgvalue-ssareg.ll b/llvm/test/DebugInfo/X86/sdag-dbgvalue-ssareg.ll
new file mode 100644
index 00000000000..dd9fac64c02
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/sdag-dbgvalue-ssareg.ll
@@ -0,0 +1,60 @@
+; RUN: llc -start-after=codegenprepare -stop-before expand-isel-pseudos -o - %s | FileCheck %s
+
+; Test that dbg.values of an SSA variable that's not used in a basic block,
+; is converted to a DBG_VALUE in that same basic block. We know that %1 is
+; live from the entry bb to the exit bb, is allocated a vreg because it's
+; used across basic blocks. We should be able to produce a DBG_VALUE that
+; refers to it in the nextbb block.
+
+source_filename = "debug.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define dso_local i32 @main(i32 %arg0, i32 %arg1) local_unnamed_addr !dbg !11 {
+entry:
+  %0 = add i32 %arg0, 42, !dbg !26
+  %1 = add i32 %arg1, 101, !dbg !26
+  %cmp = icmp eq i32 %1, 0
+  br i1 %cmp, label %nextbb, label %exit, !dbg !26
+
+nextbb:
+; CHECK-LABEL: bb.{{.*}}.nextbb
+  %2 = mul i32 %0, %arg1, !dbg !26
+; CHECK: IMUL32rr
+  call void @llvm.dbg.value(metadata i32 %1, metadata !16, metadata !DIExpression()), !dbg !27
+; CHECK-NEXT: DBG_VALUE
+  br label %exit, !dbg !26
+
+; CHECK-LABEL: bb.{{.*}}.exit
+exit:
+  %3 = phi i32 [ 12, %entry ], [ %2, %nextbb ], !dbg !26
+  %4 = add i32 %1, %3, !dbg !26
+  ret i32 %4, !dbg !26
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 7.0.0 (x)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4)
+!3 = !DIFile(filename: "debug.c", directory: "")
+!4 = !{}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 7.0.0 (x)"}
+!11 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 3, type: !12, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, unit: !2, retainedNodes: !14)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!6}
+!14 = !{!16}
+!16 = !DILocalVariable(name: "y", scope: !11, file: !3, line: 5, type: !6)
+!26 = !DILocation(line: 4, column: 7, scope: !11)
+!27 = !DILocation(line: 5, column: 7, scope: !11)
+!31 = !{!32, !32, i64 0}
+!32 = !{!"int", !33, i64 0}
+!33 = !{!"omnipotent char", !34, i64 0}
+!34 = !{!"Simple C/C++ TBAA"}