[DAGCombiner] Keep debug information when folding (sext (truncate x)) -> (sextinreg x)

[mamai]

Summary

In the case that there is a debug value on the truncate instruction and none on the sign extend, transfer it from truncate to sign extend so it does not get lost. Also adding a test case showing the debug information in IR will result in a debug value at code generation.

[dblaikie]

Generally the test case should just test the optimization - no need to go
all the way through codegen, etc.

what's the current behavior if both instructions have a location?

& why is this in an else if block? Why should it not happen if the ifs
above it are true?

[mamai]

Good questions.

In D20432#434357, @dblaikie wrote:

Generally the test case should just test the optimization - no need to go
all the way through codegen, etc.

I there a way to test only the DAGCombiner ?

what's the current behavior if both instructions have a location?

The behavior (before this patch) was that the debug loc and debug value of the truncate instruction were lost. With this patch, if the truncate has a debug value and the sign extend does not have one, the debug value and debug location of the truncate instruction will replace the ones of the sign extend and therefor they will be forwarded to the new SIGN_EXTEND_INREG.

& why is this in an else if block? Why should it not happen if the ifs
above it are true?

In the case of if/elseif, new nodes are created and they use the SDLoc(N0). So I assumed that in those case, the debug loc of N0 didn't need to be transferred to the sign extension node. But I might be wrong, and it might make sense to transfer the information even with the new nodes using the debug loc of N0. I didn't have a test case for that.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (N0.getOpcode() == ISD::TRUNCATE) {
 
     // fold (sext (truncate x)) -> (sextinreg x).
     if (!LegalOperations || TLI.isOperationLegal(ISD::SIGN_EXTEND_INREG,
                                                  N0.getValueType())) {
       if (OpBits < DestBits)
         Op = DAG.getNode(ISD::ANY_EXTEND, SDLoc(N0), VT, Op);
       else if (OpBits > DestBits)
         Op = DAG.getNode(ISD::TRUNCATE, SDLoc(N0), VT, Op);
+      else if (N0->getHasDebugValue() && !N->getHasDebugValue()) {
+        // Instead of loosing the truncate debug information, pass it to N.
+        N->setDebugLoc(N0->getDebugLoc());
+        N->setIROrder(N0->getIROrder());
+        DAG.TransferDbgValues(N0, SDValue(N, 0));
+      }
       return DAG.getNode(ISD::SIGN_EXTEND_INREG, SDLoc(N), VT, Op,
                          DAG.getValueType(N0.getValueType()));
     }
   }
 
   // fold (sext (load x)) -> (sext (truncate (sextload x)))
   // Only generate vector extloads when 1) they're legal, and 2) they are
   // deemed desirable by the target.

test/DebugInfo/ARM/dagcombine-truncate.ll

+; RUN: llc < %s | FileCheck %s
+
+;CHECK: #DEBUG_VALUE: main:bigger
+
+; Function Attrs: noinline nounwind
+define i32 @main() !dbg !4 {
+entry:
+  %call = call i32 bitcast (i32 (...)* @max to i32 (i32, i32)*)(i32 1, i32 2), !dbg !14
+  %conv = trunc i32 %call to i8, !dbg !16
+  call void @llvm.dbg.value(metadata i8 %conv, i64 0, metadata !9, metadata !17), !dbg !18
+  %conv1 = sext i8 %conv to i32, !dbg !19
+  ret i32 %conv1, !dbg !20
+}
+
+declare i32 @max(...)
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!11, !12}
+!llvm.ident = !{!13}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.9.0 (trunk) (http://llvm.org/git/llvm.git a0a8cb640e6caead7ea28b9f04a410bc86209e4f)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "main.c", directory: "/private/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 10, type: !5, isLocal: false, isDefinition: true, scopeLine: 10, isOptimized: true, variables: !8, unit: !0)
+!5 = !DISubroutineType(types: !6)
+!6 = !{!7}
+!7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!8 = !{!9}
+!9 = !DILocalVariable(name: "bigger", scope: !4, file: !1, line: 11, type: !10)
+!10 = !DIBasicType(name: "char", size: 8, align: 8, encoding: DW_ATE_signed_char)
+!11 = !{i32 2, !"Dwarf Version", i32 4}
+!12 = !{i32 2, !"Debug Info Version", i32 3}
+!13 = !{!"clang version 3.9.0 (trunk) (http://llvm.org/git/llvm.git a0a8cb640e6caead7ea28b9f04a410bc86209e4f)"}
+!14 = !DILocation(line: 11, column: 16, scope: !15)
+!15 = !DILexicalBlockFile(scope: !4, file: !1, discriminator: 1)
+!16 = !DILocation(line: 11, column: 16, scope: !4)
+!17 = !DIExpression()
+!18 = !DILocation(line: 11, column: 7, scope: !4)
+!19 = !DILocation(line: 12, column: 9, scope: !4)
+!20 = !DILocation(line: 12, column: 2, scope: !4)