[x86] Implement R_X86_64_TLSGD GD→LE TLS relaxation#1528
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Saharsh Burra (bsaharsh) wants to merge 1 commit into
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[x86] Implement R_X86_64_TLSGD GD→LE TLS relaxation#1528Saharsh Burra (bsaharsh) wants to merge 1 commit into
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Have fixed the clang format and added the fixes comment. |
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Steven Ramirez Rosa (Steven6798)
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the changes here also have GOTPCRELX, now since GOTPCRELX is merged, please rebase |
For non-preemptible symbols in executables (static, dynamic, or PIE), the 16-byte GD sequence: 66 48 8d 3d [rel32] leaq sym@tlsgd(%rip), %rdi 66 66 48 e8 [rel32] callq __tls_get_addr@PLT is rewritten in-place to the LE form: 64 48 8b 04 25 00 00 00 00 movq %fs:0x0, %rax 48 8d 80 [imm32] leaq tpoff(%rax), %rax This eliminates the 2-slot GD GOT pair, the R_X86_64_DTPMOD64 and R_X86_64_DTPOFF64 dynamic relocations, and the __tls_get_addr PLT call. Relaxation fires when: - The symbol is non-preemptible and the output is an executable (static, dynamic, or PIE) — LE requires the variable's TP-relative offset to be a link-time constant, which holds only for the executable's own block - The link is not partial (-r) GD→LE is mandatory and fires regardless of --relax, matching lld and bfd. The --relax flag governs optional relaxations (e.g. GOTPCRELX); without GD→LE, executables fail with "undefined reference to __tls_get_addr". Approach -------- - Scan: qualifying TLSGD relocations skip GOT-pair creation and are recorded as candidates. The companion call reloc at TLSGD_offset+8 (R_X86_64_PLT32 for standard sequences, R_X86_64_GOTPCRELX for -fno-plt) is tracked to suppress undefined-ref errors and skip GOT/PLT allocation for __tls_get_addr. - postProcessing: the TLSGD candidate loop always runs (mandatory). The 16-byte rewrite uses finalizeTLSSymbol() to get the TP-relative offset. - Apply: early-return OK for candidates and companions to skip the apply path, which has no GOT pair to reference. - --trace=relax emits trace_relax_tlsgd_le for each relaxed reference. Link modes ---------- Relaxation applies to static and dynamic executables and PIE, where the variable's TP-relative offset is a link-time constant. Tests: - TLSGDRelax_StaticHidden - TLSGDRelax_DynExeHidden - TLSGDRelax_PIE_Hidden - TLSGDRelax_SharedPreemptible_NoRelax - TLSGDRelax_SharedHidden_NoRelax - TLSGDRelax_PartialRelink - TLSGDRelax_Trace - TLSGDRelax_NoPLT Fixes qualcomm#1490 Signed-off-by: bsaharsh <[email protected]>
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Implements GD→LE TLS relaxation for R_X86_64_TLSGD relocations.
For non-preemptible symbols in executables, the 16-byte GD sequence is
rewritten in-place to the LE form when:
dynamic, or PIE)
GD→LE is mandatory and fires regardless of --relax, matching lld and
bfd. Without this relaxation, executables fail with "undefined reference
to __tls_get_addr".
The companion call relocation at TLSGD_offset+8 may be either
R_X86_64_PLT32 (standard) or R_X86_64_GOTPCRELX (-fno-plt). Both are
handled.
Note: This PR contains two commits. The first ([x86] Support
R_X86_64_GOTPCRELX relaxation) is a dependency
Tests:
Fixes #1490