[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style l

[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style linker
relaxation and will not be discarded by ld/objcopy --discard-locals
(-X).

In contrast, GNU assembler's riscv port assigns a fake name ".L0 " (with
a trailing space) to these symbols so that will be discarded by
ld/objcopy --discard-locals.

This patch matches the GNU behavior. Since Clang's RISC-V targets pass
-X to ld, and GNU ld defaults to -X for RISC-V targets, these ".L0 "
symbols will be discarded after linking by default, as expected by
users.

The llvm-symbolizer special case for RISC-V `SF_FormatSpecific` symbols
https://reviews.llvm.org/D98669 needs to be adjusted.

Note: `"":` in assembly currently crashes.

Note: bolt tests used /usr/bin/clang before
llvmorg-19-init-9532-g59bfc3106874.
The revert llvmorg-19-init-9531-g28b55342e1a8 actually broke
bolt/test/RISCV/fake-label-no-entry.c

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Revert "[MC] Rename temporary symbols of empty name to ".L0 " (#89693)"

This reverts commit 96c45a7fa12619c3abd6b81effe4c80f0916b78b.

Broke BOLT builders and all pre-merge testing:
https://lab.llvm

Revert "[MC] Rename temporary symbols of empty name to ".L0 " (#89693)"

This reverts commit 96c45a7fa12619c3abd6b81effe4c80f0916b78b.

Broke BOLT builders and all pre-merge testing:
https://lab.llvm.org/buildbot/#/builders/244/builds/28097

show more ...

[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style l

[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style linker
relaxation and will not be discarded by ld/objcopy --discard-locals
(-X).

In contrast, GNU assembler's riscv port assigns a fake name ".L0 " (with
a trailing space) to these symbols so that will be discarded by
ld/objcopy --discard-locals.

This patch matches the GNU behavior. Since Clang's RISC-V targets pass
-X to ld, and GNU ld defaults to -X for RISC-V targets, these ".L0 "
symbols will be discarded after linking by default, as expected by
users.

The llvm-symbolizer special case for RISC-V `SF_FormatSpecific` symbols
https://reviews.llvm.org/D98669 needs to be adjusted.

Note: `"":` in assembly currently crashes.

show more ...

Revert "[MC] Rename temporary symbols of empty name to ".L0 "" (#90002)

Reverts llvm/llvm-project#89693

This broke the premerge bot (bolt tests failing)

[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style

[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style linker
relaxation and will not be discarded by ld/objcopy --discard-locals
(-X).

In contrast, GNU assembler's riscv port assigns a fake name ".L0 " (with
a trailing space) to these symbols so that will be discarded by
ld/objcopy --discard-locals.

This patch matches the GNU behavior. Since Clang's RISC-V targets pass
-X to ld, and GNU ld defaults to -X for RISC-V targets, these ".L0 "
symbols will be discarded after linking by default, as expected by
users.

The llvm-symbolizer special case for RISC-V `SF_FormatSpecific` symbols
https://reviews.llvm.org/D98669 needs to be adjusted.

Note: `"":` in assembly currently crashes.

show more ...

[RISCV] Allow delayed decision for ADD/SUB relocations

For a label difference `A-B` in assembly, if A and B are separated by a
linker-relaxable instruction, we should emit a pair of ADD/SUB
relocati

[RISCV] Allow delayed decision for ADD/SUB relocations

For a label difference `A-B` in assembly, if A and B are separated by a
linker-relaxable instruction, we should emit a pair of ADD/SUB
relocations (e.g. R_RISCV_ADD32/R_RISCV_SUB32,
R_RISCV_ADD64/R_RISCV_SUB64).

However, the decision is made upfront at parsing time with inadequate
heuristics (`requiresFixup`). As a result, LLVM integrated assembler
incorrectly suppresses R_RISCV_ADD32/R_RISCV_SUB32 for the following
code:
```
// Simplified from a workaround https://android-review.googlesource.com/c/platform/art/+/2619609
// Both end and begin are not defined yet. We decide ADD/SUB relocations upfront and don't know they will be needed.
.4byte end-begin

begin:
call foo
end:
```

To fix the bug, make two primary changes:

* Delete `requiresFixups` and the overridden emitValueImpl (from D103539).
This deletion requires accurate evaluateAsAbolute (D153097).
* In MCAssembler::evaluateFixup, call handleAddSubRelocations to emit
ADD/SUB relocations.

However, there is a remaining issue in
MCExpr.cpp:AttemptToFoldSymbolOffsetDifference. With MCAsmLayout, we may
incorrectly fold A-B even when A and B are separated by a
linker-relaxable instruction. This deficiency is acknowledged (see
D153097), but was previously bypassed by eagerly emitting ADD/SUB using
`requiresFixups`. To address this, we partially reintroduce `canFold` (from
D61584, removed by D103539).

Some expressions (e.g. .size and .fill) need to take the `MCAsmLayout`
code path in AttemptToFoldSymbolOffsetDifference, avoiding relocations
(weird, but matching GNU assembler and needed to match user
expectation). Switch to evaluateKnownAbsolute to leverage the `InSet`
condition.

As a bonus, this change allows for the removal of some relocations for
the FDE `address_range` field in the .eh_frame section.

riscv64-64b-pcrel.s contains the main test.
Add a linker relaxable instruction to dwarf-riscv-relocs.ll to test what
it intends to test.
Merge fixups-relax-diff.ll into fixups-diff.ll.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D155357

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[llvm-readobj] Print <null> for relocation target with an empty name

For a relocation, we don't differentiate the two cases:

* the symbol index is 0
* the symbol index is non zero, the type is not

[llvm-readobj] Print <null> for relocation target with an empty name

For a relocation, we don't differentiate the two cases:

* the symbol index is 0
* the symbol index is non zero, the type is not STT_SECTION, and the name is empty. Clang generates such local symbols for RISC-V linker relaxation.

So we may print
```
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001c 0000000100000039 R_RISCV_32_PCREL 0000000000000000 0

// llvm-readobj
0x1C R_RISCV_32_PCREL - 0x0
```

while GNU readelf prints "<null>", which is clearer. Let's match the GNU behavior.
Related to https://reviews.llvm.org/D81842

```
000000000000001c 0000000100000039 R_RISCV_32_PCREL 0000000000000000 <null> + 0

// llvm-readobj
0x1C R_RISCV_32_PCREL <null> 0x0
```

Reviewed By: jhenderson, kito-cheng

Differential Revision: https://reviews.llvm.org/D155353

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[RISCV] Convert some tests to opaque pointers (NFC)

test: clean up some of the RISCV tests (NFC)

This addresses some post-commit comments from jrtc27 to make the tests
easier to process.

RISCV: adjust handling of relocation emission for RISCV

This re-architects the RISCV relocation handling to bring the
implementation closer in line with the implementation in binutils. We
would pre

RISCV: adjust handling of relocation emission for RISCV

This re-architects the RISCV relocation handling to bring the
implementation closer in line with the implementation in binutils. We
would previously aggressively resolve the relocation. With this
restructuring, we always will emit a paired relocation for any symbolic
difference of the type of S±T[±C] where S and T are labels and C is a
constant.

GAS has a special target hook controlled by `RELOC_EXPANSION_POSSIBLE`
which indicates that a fixup may be expanded into multiple relocations.
This is used by the RISCV backend to always emit a paired relocation -
either ADD[WIDTH] + SUB[WIDTH] for text relocations or SET[WIDTH] +
SUB[WIDTH] for a debug info relocation. Irrespective of whether linker
relaxation support is enabled, symbolic difference is always emitted as
a paired relocation.

This change also sinks the target specific behaviour down into the
target specific area rather than exposing it to the shared relocation
handling. In the process, we also sink the "special" handling for debug
information down into the RISCV target. Although this improves the path
for the other targets, this is not necessarily entirely ideal either.
The changes in the debug info emission could be done through another
type of hook as this functionality would be required by any other target
which wishes to do linker relaxation. However, as there are no other
targets in LLVM which currently do this, this is a reasonable thing to
do until such time as the code needs to be shared.

Improve the handling of the relocation (and add a reduced test case from
the Linux kernel) to ensure that we handle complex expressions for
symbolic difference. This ensures that we correct relocate symbols with
the adddends normalized and associated with the addition portion of the
paired relocation.

This change also addresses some review comments from Alex Bradbury about
the relocations meant for use in the DWARF CFA being named incorrectly
(using ADD6 instead of SET6) in the original change which introduced the
relocation type.

This resolves the issues with the symbolic difference emission
sufficiently to enable building the Linux kernel with clang+IAS+lld
(without linker relaxation).

Resolves PR50153, PR50156!
Fixes: ClangBuiltLinux/linux#1023, ClangBuiltLinux/linux#1143

Reviewed By: nickdesaulniers, maskray

Differential Revision: https://reviews.llvm.org/D103539

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[llvm-readobj] Update tests because of changes in llvm-readobj behavior

This patch updates tests using llvm-readobj and llvm-readelf, because
soon reading from stdin will be achievable only via a '-

[llvm-readobj] Update tests because of changes in llvm-readobj behavior

This patch updates tests using llvm-readobj and llvm-readelf, because
soon reading from stdin will be achievable only via a '-' as described
here: https://bugs.llvm.org/show_bug.cgi?id=46400. Patch with changes to
llvm-readobj behavior is here: https://reviews.llvm.org/D83704

Differential Revision: https://reviews.llvm.org/D83912

Reviewed by: jhenderson, MaskRay, grimar

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[RISCV][MC] Don't fold symbol differences if requiresDiffExpressionRelocations is true

When emitting the difference between two symbols, the standard behavior is
that the difference will be resolve

[RISCV][MC] Don't fold symbol differences if requiresDiffExpressionRelocations is true

When emitting the difference between two symbols, the standard behavior is
that the difference will be resolved to an absolute value if both of the
symbols are offsets from the same data fragment. This is undesirable on
architectures such as RISC-V where relaxation in the linker may cause the
computed difference to become invalid. This caused an issue when compiling to
object code, where the size of a function in the debug information was already
calculated even though it could change as a consequence of relaxation in the
subsequent linking stage.

This patch inhibits the resolution of symbol differences to absolute values
where the target's AsmBackend has declared that it does not want these to be
folded.

Differential Revision: https://reviews.llvm.org/D45773
Patch by Edward Jones.

llvm-svn: 339864

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