[MachineCombiner] Preserve debug instruction number

[fdeazeve]

Summary

Each target's TargetInstrInfo is responsible for announcing which code
patterns it is able to transform during the MachineCombiner pass.
Currently, these patterns are applied without preserving the debug
instruction number required by the InstrRef implementation of
LiveDebugValues. As such, we've seen a number of examples where debug
information is dropped for variables in InstrRef mode that were
otherwise available in VarLoc mode. This has been observed both in X86
and AArch examples.

This commit is an initial attempt at preserving said numbers by changing
the general (target agnostic) implementation of TargetInstrInfo: the
reassociation pattern must keep the debug number of the "top level"
instruction, i.e., the instruction whose value represents the final
value of the arithmetic expression. Intermediate values must have their
debug number dropped, as they have no equivalent value in the
unoptimized code.

Future work is required to update each target's
TargetInstrInfo::genAlternativeCodeSequence method.

[fdeazeve]

Simplify test

[fdeazeve]

Remove WIP comments.

[fdeazeve]

llvm/test/CodeGen/X86/machine-combiner-dbg.ll

1

FWIW this is heavily based on llvm/test/CodeGen/X86/machine-combiner.ll

[fdeazeve]

Fix typo in commit message

[jmorse]

LGTM; you might consider making it a MIR test that just runs the machine-combiner pass, but I don't think that'll have any material advantage over an .ll test, the only difference is making the exact input to machine-combiner explicitly visible.

[fdeazeve]

Convert test into a MIR test that _only_ runs machine combiner

[fdeazeve]

In D145759#4184187, @jmorse wrote:

LGTM; you might consider making it a MIR test that just runs the machine-combiner pass

Agreed! This makes the test less susceptible to changes elsewhere in the backend.

llvm/lib/CodeGen/TargetInstrInfo.cpp

void TargetInstrInfo::reassociateOps(
 
   setSpecialOperandAttr(Root, Prev, *MIB1, *MIB2);
 
   // Record new instructions for insertion and old instructions for deletion.
   InsInstrs.push_back(MIB1);
   InsInstrs.push_back(MIB2);
   DelInstrs.push_back(&Prev);
   DelInstrs.push_back(&Root);
+
+  // We transformed:
+  // B = A op X (Prev)
+  // C = B op Y (Root)
+  // Into:
+  // B = X op Y (MIB1)
+  // C = A op B (MIB2)
+  // C has the same value as before, B doesn't; as such, keep the debug number
+  // of C but not of B.
+  if (unsigned OldRootNum = Root.peekDebugInstrNum())
+    MIB2.getInstr()->setDebugInstrNum(OldRootNum);
 }
 
 void TargetInstrInfo::genAlternativeCodeSequence(
     MachineInstr &Root, MachineCombinerPattern Pattern,
     SmallVectorImpl<MachineInstr *> &InsInstrs,
     SmallVectorImpl<MachineInstr *> &DelInstrs,
     DenseMap<unsigned, unsigned> &InstIdxForVirtReg) const {
   MachineRegisterInfo &MRI = Root.getMF()->getRegInfo();

llvm/test/CodeGen/X86/machine-combiner-dbg.ll

+; RUN: llc -mtriple=x86_64-gnu-linux -stop-after=machine-combiner < %s | FileCheck %s
+
+define float @reassoc_me(float %f1, float %f2, float %f3, float %f4) !dbg !9 {
+  %add = fadd reassoc nsz float %f1, %f2, !dbg !21
+  %add1 = fadd reassoc nsz float %add, %f3, !dbg !21
+  %add2 = fadd reassoc nsz float %add1, %f4, !dbg !21
+  call void @llvm.dbg.value(metadata float %add2, metadata !20, metadata !DIExpression()), !dbg !21
+  ret float %add2, !dbg !21
+}
+
+; CHECK: ![[VAR:.*]] = !DILocalVariable(name: "temp3"
+
+; CHECK:      %[[arg3:.*]]:fr32 = COPY $xmm3
+; CHECK-NEXT: %[[arg2:.*]]:fr32 = COPY $xmm2
+; CHECK-NEXT: %[[arg1:.*]]:fr32 = COPY $xmm1
+; CHECK-NEXT: %[[arg0:.*]]:fr32 = COPY $xmm0
+; CHECK-NEXT: %[[add1:.*]]:fr32 = {{.*}} ADDSSrr %[[arg0]], %[[arg1]]
+; CHECK-NEXT: %[[add2:.*]]:fr32 = {{.*}} ADDSSrr %[[arg2]], %[[arg3]]
+; CHECK-NEXT:                            ADDSSrr %[[add1]], killed %[[add2]], {{.*}} debug-instr-number 1
+; CHECK-NEXT: DBG_INSTR_REF ![[VAR]], !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0)
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "hello", isOptimized: true, emissionKind: FullDebug)
+!1 = !DIFile(filename: "reassoc.c", directory: "blah")
+!2 = !{i32 7, !"Dwarf Version", i32 4}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = distinct !DISubprogram(name: "reassoc_me", scope: !1, file: !1, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !12, !12, !12, !12}
+!12 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!13 = !{!20}
+!20 = !DILocalVariable(name: "temp3", scope: !9, file: !1, line: 4, type: !12)
+!21 = !DILocation(line: 0, scope: !9)