Suppress spurious warnings due to R_RISCV_SET_ULEB128

[MaskRay]

llvm-objdump -S issues unnecessary warnings for RISC-V relocatable files
containing .debug_loclists or .debug_rnglists sections with ULEB128
relocations. This occurred because DWARFObjInMemory verifies support for all
relocation types, triggering warnings for unsupported ones.

% llvm-objdump -S a.o
...
0000000000000000 <foo>:
warning: failed to compute relocation: R_RISCV_SUB_ULEB128, Invalid data was encountered while parsing the file
warning: failed to compute relocation: R_RISCV_SET_ULEB128, Invalid data was encountered while parsing the file
...

This change fixes #101544 by declaring support for the two ULEB128
relocation types, silencing the spurious warnings.

---

In DWARF v5 builds, DW_LLE_offset_pair/DW_RLE_offset_pair might be
generated in .debug_loclists/.debug_rnglists with ULEB128 relocations.
They are only read by llvm-dwarfdump to dump section content and verbose
DW_AT_location/DW_AT_ranges output for relocatable files.

The DebugInfoDWARF user (e.g. DWARFDebugRnglists.cpp) calls
Data.getULEB128 without checking the ULEB128 relocations, as the
unrelocated value holds meaning (refer to the assembler
implementation https://reviews.llvm.org/D157657). This differs from
.quad .Lfoo, which requires relocation reading (e.g.
https://reviews.llvm.org/D74404).

[llvmbot]

@llvm/pr-subscribers-llvm-binary-utilities

Author: Fangrui Song (MaskRay)

Changes

llvm-objdump -S prints spurious warnings on RISC-V relocatable files
when .debug_loclists/.debug_rnglists with ULEB128 relocations are
present. This is because DWARFObjInMemory constructed by llvm-objdump
-S checks whether all relocation types are supported, and prints
warnings if not.

% llvm-objdump -S a.o
...
0000000000000000 <foo>:
warning: failed to compute relocation: R_RISCV_SUB_ULEB128, Invalid data was encountered while parsing the file
warning: failed to compute relocation: R_RISCV_SET_ULEB128, Invalid data was encountered while parsing the file
...

Fix #101544: we can declare support for the two ULEB128 relocation types
without an (unreachable) implementation to suppress the spurious
warnings.

---

In DWARF v5 builds, DW_LLE_offset_pair/DW_RLE_offset_pair might be
generated in .debug_loclists/.debug_rnglists with ULEB128 relocations.
They are only read by llvm-dwarfdump to dump section content and verbose
DW_AT_location/DW_AT_ranges output for relocatable files.

The DebugInfoDWARF user (e.g. DWARFDebugRnglists.cpp), calls
Data.getULEB128 without checking the ULEB128 relocations, since the
unrelocated value is already meaningful (refer to the assembler
implementation https://reviews.llvm.org/D157657). This is unlike .quad .Lfoo, which requires reading the relocation. (e.g.
https://reviews.llvm.org/D74404).

---

Full diff: https://github.com/llvm/llvm-project/pull/101607.diff

2 Files Affected:

• (modified) llvm/lib/Object/RelocationResolver.cpp (+6)
• (added) llvm/test/tools/llvm-objdump/ELF/RISCV/source-interleave.ll (+115)

diff --git a/llvm/lib/Object/RelocationResolver.cpp b/llvm/lib/Object/RelocationResolver.cpp
index 564d9da78e97d..d9bb8f175ef83 100644
--- a/llvm/lib/Object/RelocationResolver.cpp
+++ b/llvm/lib/Object/RelocationResolver.cpp
@@ -457,6 +457,12 @@ static bool supportsRISCV(uint64_t Type) {
   case ELF::R_RISCV_SUB32:
   case ELF::R_RISCV_ADD64:
   case ELF::R_RISCV_SUB64:
+    // Because the unrelocated value generated by .uleb128 A-B (used by
+    // loclists/rnglists) is meaningful, DebugInfoDWARF does not inspect the
+    // relocations. We declare support for the two relocation types without an
+    // (unreachable) implementation.
+  case ELF::R_RISCV_SET_ULEB128:
+  case ELF::R_RISCV_SUB_ULEB128:
     return true;
   default:
     return false;
diff --git a/llvm/test/tools/llvm-objdump/ELF/RISCV/source-interleave.ll b/llvm/test/tools/llvm-objdump/ELF/RISCV/source-interleave.ll
new file mode 100644
index 0000000000000..e5bcaf80d699d
--- /dev/null
+++ b/llvm/test/tools/llvm-objdump/ELF/RISCV/source-interleave.ll
@@ -0,0 +1,115 @@
+; RUN: rm -rf %t && split-file %s %t && cd %t
+; RUN: llc -filetype=obj a.ll -o a.o
+; RUN: llvm-readelf -r a.o 2>err | FileCheck %s --check-prefix=RELOC
+; RUN: llvm-objdump -Sl --no-show-raw-insn a.o 2>err | FileCheck %s
+;; Test that ULEB128 relocs do not lead to spurious warnings. #101544
+; RUN: count 0 < err
+
+; RELOC:      Relocation section '.rela.debug_loclists'
+; RELOC:      R_RISCV_SET_ULEB128
+; RELOC-NEXT: R_RISCV_SUB_ULEB128
+
+; CHECK:      ; foo():
+; CHECK-NEXT: /proc/self/cwd/a.c:2
+; CHECK-NEXT: ; int foo(int x) {
+; CHECK-NEXT:   0: addi    sp, sp, -0x10
+; CHECK-NEXT:   2: sd      ra, 0x8(sp)
+; CHECK-NEXT: /proc/self/cwd/a.c:3
+; CHECK-NEXT: ; ext();
+; CHECK-NEXT:   4: auipc   ra, 0x0
+
+;--- a.c
+int ext(void);
+int foo(int x) {
+  ext();
+  return 0;
+}
+
+int ext(void);
+void foo2() {
+  {
+    int ret = ext();
+    if (__builtin_expect(ret, 0))
+      ext();
+  }
+}
+;--- gen
+clang --target=riscv64-linux -S -emit-llvm -g -O1 a.c -o - | sed -E '/^attribute/s/,-[-0-9a-z]+//g'
+;--- a.ll
+; ModuleID = 'a.c'
+source_filename = "a.c"
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "riscv64-unknown-linux"
+
+; Function Attrs: nounwind uwtable
+define dso_local noundef signext i32 @foo(i32 noundef signext %x) local_unnamed_addr #0 !dbg !13 {
+entry:
+    #dbg_value(i32 %x, !18, !DIExpression(), !19)
+  %call = tail call signext i32 @ext() #2, !dbg !20
+  ret i32 0, !dbg !21
+}
+
+declare !dbg !22 signext i32 @ext() local_unnamed_addr #1
+
+; Function Attrs: nounwind uwtable
+define dso_local void @foo2() local_unnamed_addr #0 !dbg !25 {
+entry:
+  %call = tail call signext i32 @ext() #2, !dbg !31
+    #dbg_value(i32 %call, !29, !DIExpression(), !32)
+  %tobool.not = icmp eq i32 %call, 0, !dbg !33
+  br i1 %tobool.not, label %if.end, label %if.then, !dbg !35, !prof !36
+
+if.then:                                          ; preds = %entry
+  %call1 = tail call signext i32 @ext() #2, !dbg !37
+  br label %if.end, !dbg !37
+
+if.end:                                           ; preds = %if.then, %entry
+  ret void, !dbg !38
+}
+
+attributes #0 = { nounwind uwtable "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="generic-rv64" "target-features"="+64bit,+a,+c,+d,+f,+m,+relax,+zicsr,+zmmul" }
+attributes #1 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="generic-rv64" "target-features"="+64bit,+a,+c,+d,+f,+m,+relax,+zicsr,+zmmul" }
+attributes #2 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !8, !9, !10, !11, !12}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "a.c", directory: "/proc/self/cwd", checksumkind: CSK_MD5, checksum: "4791066d0b0e4fd9c4b4df1c56f349cb")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 1, !"target-abi", !"lp64d"}
+!6 = !{i32 6, !"riscv-isa", !7}
+!7 = !{!"rv64i2p1_m2p0_a2p1_f2p2_d2p2_c2p0_zicsr2p0_zmmul1p0"}
+!8 = !{i32 8, !"PIC Level", i32 2}
+!9 = !{i32 7, !"PIE Level", i32 2}
+!10 = !{i32 7, !"uwtable", i32 2}
+!11 = !{i32 8, !"SmallDataLimit", i32 8}
+!12 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
+!13 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 2, type: !14, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !17)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!16, !16}
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !{!18}
+!18 = !DILocalVariable(name: "x", arg: 1, scope: !13, file: !1, line: 2, type: !16)
+!19 = !DILocation(line: 0, scope: !13)
+!20 = !DILocation(line: 3, column: 3, scope: !13)
+!21 = !DILocation(line: 4, column: 3, scope: !13)
+!22 = !DISubprogram(name: "ext", scope: !1, file: !1, line: 1, type: !23, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!23 = !DISubroutineType(types: !24)
+!24 = !{!16}
+!25 = distinct !DISubprogram(name: "foo2", scope: !1, file: !1, line: 8, type: !26, scopeLine: 8, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !28)
+!26 = !DISubroutineType(types: !27)
+!27 = !{null}
+!28 = !{!29}
+!29 = !DILocalVariable(name: "ret", scope: !30, file: !1, line: 10, type: !16)
+!30 = distinct !DILexicalBlock(scope: !25, file: !1, line: 9, column: 3)
+!31 = !DILocation(line: 10, column: 15, scope: !30)
+!32 = !DILocation(line: 0, scope: !30)
+!33 = !DILocation(line: 11, column: 9, scope: !34)
+!34 = distinct !DILexicalBlock(scope: !30, file: !1, line: 11, column: 9)
+!35 = !DILocation(line: 11, column: 9, scope: !30)
+!36 = !{!"branch_weights", !"expected", i32 2000, i32 1}
+!37 = !DILocation(line: 12, column: 7, scope: !34)
+!38 = !DILocation(line: 14, column: 1, scope: !25)

[jh7370]

One minor point. Probably best if one of the RISCV people take a look at this though.

[jh7370]

Worth a link rather than #101544? Without the immediate context, I wouldn't easily recognise what the current reference is supposed to be.

[MaskRay]

Switched to https://github.com/llvm/llvm-project/issues/101544 instead

[MaskRay]

@kito-cheng :)

[kito-cheng]

LGTM from the RISC-V side :)