[x86] Implement R_X86_64_GOTTPOFF IE→LE TLS relaxation#1547
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Saharsh Burra (bsaharsh) wants to merge 1 commit into
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[x86] Implement R_X86_64_GOTTPOFF IE→LE TLS relaxation#1547Saharsh Burra (bsaharsh) wants to merge 1 commit into
Saharsh Burra (bsaharsh) wants to merge 1 commit into
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Saharsh Burra (bsaharsh)
requested review from
Parth (parth-07) and
Shankar Easwaran (quic-seaswara)
as code owners
July 22, 2026 04:37
For non-preemptible symbols in executables (static, dynamic, or PIE), rewrite GOTTPOFF GOT-indirect accesses to direct TP-relative immediates: 48 8b 05 [rel32] movq sym@gottpoff(%rip), %rax -> movq $tpoff, %rax 48 03 05 [rel32] addq sym@gottpoff(%rip), %rax -> leaq tpoff(%rax), %rax 48 03 25 [rel32] addq sym@gottpoff(%rip), %rsp -> addq $tpoff, %rsp All six MOV/ADD sub-cases are 7→7 bytes (size-preserving). %rsp and %r12 use addq $imm instead of leaq, which would need a SIB byte (8 bytes). This eliminates one IE GOT slot (8 bytes), the R_X86_64_TPOFF64 dynamic relocation (in non-static executable links), and one runtime memory load per TLS access. 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) IE→LE is a mandatory ABI TLS transition and fires regardless of --relax, matching lld and bfd. The --relax flag governs optional relaxations (e.g. GOTPCRELX). Shared libraries are never relaxed even for hidden symbols — the loader places the DSO TLS block at a runtime-determined offset. Approach -------- - Scan: shouldRelaxTLSIEToLE() combines isTLSIERelaxable() (addend == -4, MOV/ADD opcode, RegionFragment) with the non-preemptible-in-executable condition. Qualifying GOTTPOFF relocations skip GOT creation and are recorded as candidates. - postProcessing: the IE→LE candidate loop always runs (mandatory). relaxTLSIEReloc() patches the REX/opcode/ModR/M bytes and writes the immediate. - Apply: relocGOTRelative early-returns OK for candidates, whose bytes are already rewritten and which have no GOT slot to reference. - --trace=relax emits trace_relax_gottpoff for each relaxed reference. Link modes ---------- IE→LE applies to static, dynamic, and PIE executables for non-preemptible symbols. Shared objects and preemptible symbols keep the GOT slot with the R_X86_64_TPOFF64 dynamic reloc. Partial links (-r) are never relaxed. Tests: - TLSIERelax_StaticMov - TLSIERelax_StaticAdd - TLSIERelax_StaticAddRsp - TLSIERelax_ExecNonPreemptible - TLSIERelax_SharedKeepsGOT - TLSIERelax_AddendNonM4 - TLSIERelax_PartialRelink - TLSIERelax_Trace Fixes qualcomm#1511 Signed-off-by: bsaharsh <[email protected]>
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Implements IE→LE TLS relaxation for R_X86_64_GOTTPOFF relocations.
For non-preemptible symbols in executables, GOTTPOFF GOT-indirect accesses
are rewritten in-place to direct TP-relative immediates when:
dynamic, or PIE)
All MOV and ADD sub-cases are handled (7→7 bytes, size-preserving). %rsp
and %r12 use addq $imm instead of leaq since leaq would need a SIB byte
(8 bytes).
IE→LE is mandatory and fires regardless of --relax, matching lld and bfd.
Without this relaxation, the GOT slot and R_X86_64_TPOFF64 dynamic
relocation are unnecessarily kept in executables.
Note: This PR contains two commits. The first ([x86] Support
R_X86_64_GOTPCRELX relaxation) is a dependency.
Tests:
TLSIERelax_StaticMov
TLSIERelax_StaticAdd
TLSIERelax_StaticAddRsp
TLSIERelax_ExecNonPreemptible
TLSIERelax_SharedKeepsGOT
TLSIERelax_AddendNonM4
TLSIERelax_PartialRelink
TLSIERelax_Trace
Fixes #1511