riscv: link libgcc for jemalloc's __ffsdi2

[kali]

Base rv64gc has no count-trailing-zeros instruction, so gcc lowers the ffs in bit_util.h to a __ffsdi2 libgcc call. rust's compiler_builtins ships the rest of the *di2 family but not that one, so linking a static riscv64 binary fails with undefined reference to `__ffsdi2'.

Name libgcc for riscv as is already done for android, guarded on a gnu-like compiler so a clang/compiler-rt toolchain is left alone.

Summary by CodeRabbit

• Bug Fixes
  • Improved builds for RISC-V targets using GNU-like compilers by ensuring the required compiler support library is linked.
  • Resolved linker failures on base RV64GC systems lacking the Zbb extension.

[ti-chi-bot]

Welcome @kali! It looks like this is your first PR to tikv/jemallocator 🎉

[coderabbitai]

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• jemalloc-sys/build.rs

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📝 Walkthrough

Walkthrough

The RISC-V build script now links gcc when it detects a GNU-like compiler. This supplies __ffsdi2 for rv64gc builds while retaining the existing atomic linkage.

Changes

RISC-V linker configuration

Layer / File(s)	Summary
GNU libgcc linkage jemalloc-sys/build.rs	The RISC-V branch checks compiler.is_like_gnu() and links gcc when the condition is true.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 0a020

GNU RISC-V builds now link libgcc to resolve the missing __ffsdi2 symbol for base rv64gc targets, without changing Clang/compiler-rt linkage. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 5

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Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: linking libgcc for jemalloc's RISC-V __ffsdi2 dependency.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[BusyJay]

Do you have a minimal reproduction? It seems rustc will link gcc automatically (tested on Rust 1.71 和 Rust 1.95).

[kali]

Yes. Sorry, there was a missing bit. The discriminator is a static link: with -C target-feature=+crt-static rustc passes -nodefaultlibs and, on musl, uses its self-contained rlibs (libunwind, compiler_builtins), so nothing names libgcc. A default dynamic build links libgcc_s.so.1, which exports __ffsdi2 — that is why it looks like rustc links gcc automatically.

Minimal repro, riscv64gc-unknown-linux-musl, rustc 1.98.0, Bootlin riscv64-lp64d--musl--stable-2025.08-1 (gcc 14.3.0), stock tikv-jemallocator 0.7.0 / tikv-jemalloc-sys 0.7.1 from crates.io.

TC comes from https://toolchains.bootlin.com/downloads/releases/toolchains/riscv64-lp64d/tarballs/riscv64-lp64d--musl--stable-2025.08-1.tar.xz .

# Cargo.toml
[dependencies]
tikv-jemallocator = "0.7"

// src/main.rs
#[global_allocator]
static A: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
fn main() { println!("{}", vec![1u8; 32].len()); }

TC=<bootlin-toolchain>
export CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_MUSL_LINKER=$TC/bin/riscv64-buildroot-linux-musl-gcc
export CC_riscv64gc_unknown_linux_musl=$TC/bin/riscv64-buildroot-linux-musl-gcc
export AR_riscv64gc_unknown_linux_musl=$TC/bin/riscv64-buildroot-linux-musl-ar
export CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_MUSL_RUSTFLAGS="-C target-feature=+crt-static"
cargo build --release --target riscv64gc-unknown-linux-musl

ld: libtikv_jemalloc_sys-*.rlib(bin.pic.o): in function `cfs_lu':
    .../include/jemalloc/internal/bit_util.h:290:(.text._rjem_je_bin_slab_reg_alloc_batch+0x132):
    undefined reference to `__ffsdi2'

(5 such references, all from bin_slab_reg_alloc_batch.)

Same command with -C target-feature=-crt-static links fine, and readelf -d on that binary shows NEEDED libgcc_s.so.1.

Symbol side of it:

$ nm --defined-only .../riscv64gc-unknown-linux-musl/lib/libcompiler_builtins-*.rlib | grep -oE '__[a-z]+[sd]i2' | sort -u
__absvdi2 __absvsi2 __bswapdi2 __bswapsi2 __clzdi2 __clzsi2 __cmpdi2 __ctzdi2 __ctzsi2
__negdi2 __negvdi2 __negvsi2 __paritydi2 __paritysi2 __popcountdi2 __popcountsi2 __ucmpdi2
$ nm --defined-only $(riscv64-buildroot-linux-musl-gcc -print-libgcc-file-name) | grep ffsdi2
0000000000000000 T __ffsdi2

So compiler_builtins carries the rest of the family but not __ffsdi2, and gcc emits a call to it for the ffs in bit_util.h because base rv64gc has no ctz (that is Zbb).

With this PR's build.rs applied ([patch.crates-io] tikv-jemalloc-sys = { path = ... }), the +crt-static build links and the binary runs correctly under qemu-riscv64 — no -C link-arg=-lgcc needed on the user side.

[BusyJay]

LGTM