[DebugInfo][ARM] Not readonly check for RWPI globals

[keith.walker.arm]

Summary

When compiling for the RWPI relocation model [1], the debug information is wrong for readonly global variables.

Writable global variables are accessed by the static base register (R9 on ARM) in the RWPI relocation model. This is being correctly generated

Readonly global variables are not accessed by the static base register in the RWPI relocation model. This case is incorrectly generating the same debugging information as for writable global variables.

References:
[1] ARM Read-Write Position Independence: https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#read-write-position-independence-rwpi

[keith.walker.arm]

The failing pre-commit tests:

• ThreadSanitizer-x86_64::restore_stack.cpp
• libFuzzer::fuzzer-leak.test

are unrelated to this change (they do not involve any RWPI code)

[amilendra]

llvm/test/DebugInfo/ARM/ropi.ll

21

I think these version specific content are not needed, and could break after the next version is branched?

22

Environment specific content; could potentially fail on Windows?

34

Do we need to check for these?
I think this test case can be considerably reduced to use only the bare minimum needed?

[amilendra]

llvm/test/DebugInfo/ARM/ropi.ll

21

Ah wait. These are fixed information so no specific to the commit or environment.

[keith.walker.arm]

llvm/test/DebugInfo/ARM/ropi.ll

21

Yes. This is just information about the version of the build compiler used to generate the initial .ll file and is fixed.

22

This file information is just used to generate the compilation information in the generatede debugging information.
As we are not going to be actually using the debug information to actually debug on the host platform this should not be an issue.

34

I'll remove some of the entries that are not required which should make the ropi.ll test file more similar to the existing rwpi.ll test file.
The additional entries are because I used a newer version of the compiler to generate the initial ropi.ll file.

[keith.walker.arm]

Removed unnecessary lines from test case.

[keith.walker.arm]

Once again there are unrelated 8 failures showing on the pre-commit tests due to test timeouts.

[amilendra]

LGTM.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

if (Global) {
             addUInt(*Loc, dwarf::DW_FORM_udata,
                     DD->getAddressPool().getIndex(Sym, /* TLS */ true));
           }
           // 3) followed by an OP to make the debugger do a TLS lookup.
           addUInt(*Loc, dwarf::DW_FORM_data1,
                   DD->useGNUTLSOpcode() ? dwarf::DW_OP_GNU_push_tls_address
                                         : dwarf::DW_OP_form_tls_address);
         }
-      } else if (Asm->TM.getRelocationModel() == Reloc::RWPI ||
-                 Asm->TM.getRelocationModel() == Reloc::ROPI_RWPI) {
+      } else if ((Asm->TM.getRelocationModel() == Reloc::RWPI ||
+                  Asm->TM.getRelocationModel() == Reloc::ROPI_RWPI) &&
+                 !Asm->getObjFileLowering()
+                      .getKindForGlobal(Global, Asm->TM)
+                      .isReadOnly()) {
         auto FormAndOp = GetPointerSizedFormAndOp();
         // Constant
         addUInt(*Loc, dwarf::DW_FORM_data1, FormAndOp.Op);
         // Relocation offset
         addExpr(*Loc, FormAndOp.Form,
                 Asm->getObjFileLowering().getIndirectSymViaRWPI(Sym));
         // Base register
         Register BaseReg = Asm->getObjFileLowering().getStaticBase();

llvm/test/DebugInfo/ARM/ropi.ll

+; RUN: llc -mtriple armv7-linux -relocation-model=ropi -o - %s | FileCheck %s
+; RUN: llc -mtriple armv7-linux -relocation-model=ropi-rwpi -o - %s | FileCheck %s
+
+@global = constant i32 -1414812757, align 4, !dbg !0
+
+; CHECK:        .section        .debug_info
+; CHECK:        .byte 2 @ DW_AT_location
+; DW_OP_addrx 0x0
+; CHECK-NEXT:   .byte 161
+; CHECK-NEXT:   .byte 0
+
+; CHECK:        .section        .debug_addr
+; CHECK:        .long   global
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!8, !9, !10, !11, !12, !13, !14, !15, !16}
+!llvm.ident = !{!17}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "global", scope: !2, file: !5, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 15.0.0 (https://github.com/llvm/llvm-project.git 3948962b454022c2c8de6f67942a9cbd1f0351a0)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "/tmp/ropi.c", directory: "/tmp")
+!4 = !{!0}
+!5 = !DIFile(filename: "ropi.c", directory: "/tmp")
+!6 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !7)
+!7 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!8 = !{i32 7, !"Dwarf Version", i32 5}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{i32 1, !"wchar_size", i32 4}
+!11 = !{i32 1, !"min_enum_size", i32 4}
+!12 = !{i32 8, !"branch-target-enforcement", i32 0}
+!13 = !{i32 8, !"sign-return-address", i32 0}
+!14 = !{i32 8, !"sign-return-address-all", i32 0}
+!15 = !{i32 8, !"sign-return-address-with-bkey", i32 0}
+!16 = !{i32 7, !"frame-pointer", i32 2}
+!17 = !{!"clang version 15.0.0 (https://github.com/llvm/llvm-project.git 3948962b454022c2c8de6f67942a9cbd1f0351a0)"}
+