[InstrRef] Preserve debug instr num in aarch64-cond-br-tuning.

[rastogishubham]

The aarch64-cond-br-tuning pass transforms a CBZX instruction into a conditional branch (B.cond). One of the by products of the transformation is that the source instruction of the CBZX, which is an ANDXri instruction, gets transformed into a ANDSXri instruction, however this transformation doesn't preserve it's debug instruction number.

This patch fixes that issue.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: Shubham Sandeep Rastogi (rastogishubham)

Changes

The aarch64-cond-br-tuning pass transforms a CBZX instruction into a conditional branch (B.cond). One of the by products of the transformation is that the source instruction of the CBZX, which is an ANDXri instruction, gets transformed into a ANDSXri instruction, however this transformation doesn't preserve it's debug instruction number.

This patch fixes that issue.

---

Full diff: https://github.com/llvm/llvm-project/pull/132081.diff

2 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64CondBrTuning.cpp (+6)
• (added) llvm/test/CodeGen/AArch64/cond-br-tuning-instr-ref.ll (+55)

diff --git a/llvm/lib/Target/AArch64/AArch64CondBrTuning.cpp b/llvm/lib/Target/AArch64/AArch64CondBrTuning.cpp
index da72e35a248eb..a091ab45c7737 100644
--- a/llvm/lib/Target/AArch64/AArch64CondBrTuning.cpp
+++ b/llvm/lib/Target/AArch64/AArch64CondBrTuning.cpp
@@ -102,6 +102,12 @@ MachineInstr *AArch64CondBrTuning::convertToFlagSetting(MachineInstr &MI,
 
   MachineInstrBuilder MIB = BuildMI(*MI.getParent(), MI, MI.getDebugLoc(),
                                     TII->get(NewOpc), NewDestReg);
+
+  // If the MI has a debug instruction number, preserve that in the new Machine
+  // Instruction that is created.
+  if (MI.peekDebugInstrNum() != 0)
+    MIB->setDebugInstrNum(MI.peekDebugInstrNum());
+
   for (const MachineOperand &MO : llvm::drop_begin(MI.operands()))
     MIB.add(MO);
 
diff --git a/llvm/test/CodeGen/AArch64/cond-br-tuning-instr-ref.ll b/llvm/test/CodeGen/AArch64/cond-br-tuning-instr-ref.ll
new file mode 100644
index 0000000000000..19b55113572fc
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/cond-br-tuning-instr-ref.ll
@@ -0,0 +1,55 @@
+; RUN: rm -rf %t && mkdir -p %t
+; RUN: llc -O2 -mtriple=aarch64-unknown-linux-gnu --filetype=obj -o %t/file.o %s
+; RUN: llvm-dwarfdump %t/file.o -name=_ZN4llvmlsERNS_11raw_ostreamERKNS_5ErrorE -c | FileCheck %s
+
+; This testcase was obtained by looking at FileCheck.cpp and reducing it down via llvm-reduce
+
+; CHECK: DW_TAG_variable
+; CHECK-NEXT: DW_AT_location	(0x{{[0-9a-f]+}}: 
+; CHECK-NEXT: [0x{{[0-9a-f]+}}, 0x{{[0-9a-f]+}}): DW_OP_reg{{[0-9]+}} W{{[0-9]+}})
+; CHECK-NEXT: DW_AT_name	("P")
+
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-n32:64-S128-Fn32"
+target triple = "arm64-apple-macosx15.0.0"
+
+define ptr @_ZNK4llvm5Error6getPtrEv(ptr %this) {
+entry:
+  %0 = ptrtoint ptr %this to i64
+  %and = and i64 %0, -2
+  %1 = inttoptr i64 %and to ptr
+  ret ptr %1
+}
+
+define ptr @_ZN4llvmlsERNS_11raw_ostreamERKNS_5ErrorE(ptr %E) !dbg !4 {
+entry:
+  %call = call ptr @_ZNK4llvm5Error6getPtrEv(ptr %E), !dbg !13
+    #dbg_value(ptr %call, !9, !DIExpression(), !14)
+  %tobool.not = icmp eq ptr %call, null
+  br i1 %tobool.not, label %if.else, label %if.then
+
+if.then:                                          ; preds = %entry
+  store volatile ptr null, ptr %call, align 8
+  unreachable
+
+if.else:                                          ; preds = %entry
+  ret ptr null
+}
+
+!llvm.module.flags = !{!0}
+!llvm.dbg.cu = !{!1}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !2, producer: "clang version 21.0.0git (\0A\0A\0Agit@github.com:llvm/llvm-project.git 6be6400848eeec027d0cca0662c105683bcc896b\0A\0A\0A)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, retainedTypes: !3, globals: !3, imports: !3, splitDebugInlining: false, nameTableKind: Apple, sysroot: "/Library/Developer/CommandLineTools/SDKs/MacOSX15.3.sdk", sdk: "MacOSX15.3.sdk")
+!2 = !DIFile(filename: "/Users/shubhamrastogi/Development/llvm-project-instr-ref/llvm-project/llvm/lib/FileCheck/FileCheck.cpp", directory: "/Users/shubhamrastogi/Development/llvm-project-instr-ref/llvm-project/build-instr-ref-stage2", checksumkind: CSK_MD5, checksum: "ac1d2352ab68b965fe7993c780cf92d7")
+!3 = !{}
+!4 = distinct !DISubprogram(name: "operator<<", linkageName: "_ZN4llvmlsERNS_11raw_ostreamERKNS_5ErrorE", scope: !6, file: !5, line: 320, type: !7, scopeLine: 320, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !1, retainedNodes: !8)
+!5 = !DIFile(filename: "llvm/include/llvm/Support/Error.h", directory: "/Users/shubhamrastogi/Development/llvm-project-instr-ref/llvm-project", checksumkind: CSK_MD5, checksum: "f166cdaeb719f8f71fbae8128cde93e4")
+!6 = !DINamespace(name: "llvm", scope: null)
+!7 = distinct !DISubroutineType(types: !3)
+!8 = !{!9}
+!9 = !DILocalVariable(name: "P", scope: !10, file: !5, line: 321, type: !11)
+!10 = distinct !DILexicalBlock(scope: !4, file: !5, line: 321, column: 15)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "ErrorInfoBase", scope: !6, file: !5, line: 44, size: 64, flags: DIFlagTypePassByReference | DIFlagNonTrivial, elements: !3, vtableHolder: !12, identifier: "_ZTSN4llvm13ErrorInfoBaseE")
+!13 = !DILocation(line: 321, column: 21, scope: !10)
+!14 = !DILocation(line: 0, scope: !10)

[rastogishubham]

The reason I have not included a mir test that just runs the aarch64-cond-br-tuning pass on the mir and checks for the debug instruction number is:

Just because a pass runs in the pass pipeline doesn't mean it is registered in the pass registry, if we look at the code in llvm/lib/Target/AArch64/AArch64TargetMachine.cpp in function AArch64PassConfig::addILPOpts all the passes added to the passmanager here are not registered in the pass registry, so you will see them run, but if you want to use -stop-after -stop-before -run-pass etc options, you cannot do that, I don't have a rationale for why this is the case.

Therefore the test is an llvm IR and dwarfdump test that checks to make sure that the debug location does indeed survive

[rastogishubham]

#132087 contains the patch to add the aarch64-cond-br-tuning pass to the pass registry, if it is accepted, I can modify the test to be a MIR one with -run-pass

[jmorse]

LGTM; please do ad a comment into the test about what it's targeting and what behaviour is being tested, that'll save a future reader having to dig through git history