Updated in 2024-05.
Back in 2019, I studied a bit about RISC-V and filed Support Thread-Local Storage Descriptors (TLSDESC). Last year, Tatsuyuki Ishi added a specification for TLSDESC.
LLVM
On the LLVM side, the RISC-V TLSDESC work has been completed.
The the most important patch is [RISCV] Support
Global Dynamic TLSDESC in the RISC-V backend by Paul Kirth. The linker patch
by me is also significant. Furthermore, Clang requires a -mtls-dialect=
patch.
These patches are expected to be included in the upcoming LLVM 18.1
release. To obtain TLSDESC code sequences, compile your program with
clang --target=riscv64-linux -fpic -mtls-dialect=desc.
GCC
RISC-V: Implement TLS Descriptors. landed in April 2024.
binutils
RISC-V: Initial ld.bfd support for TLSDESC. landed in February 2024.
glibc
Latest patch: https://inbox.sourceware.org/libc-alpha/20230914084033.222120-1-ishitatsuyuki@gmail.com/
musl
musl added support in February 2024.
Bionic
No patch yet.
Testing
The LLVM patches need testing. Unfortunately, I didn't have a RISC-V image at hand, so I used qemu-user.
Patch musl per Re: Draft
riscv64 TLSDESC implementation 1
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63diff --git c/arch/riscv64/reloc.h w/arch/riscv64/reloc.h
index 1ca13811..7c7c0611 100644
--- c/arch/riscv64/reloc.h
+++ w/arch/riscv64/reloc.h
#define REL_DTPMOD R_RISCV_TLS_DTPMOD64
#define REL_DTPOFF R_RISCV_TLS_DTPREL64
#define REL_TPOFF R_RISCV_TLS_TPREL64
+#define REL_TLSDESC R_RISCV_TLSDESC
#define CRTJMP(pc,sp) __asm__ __volatile__( \
"mv sp, %1 ; jr %0" : : "r"(pc), "r"(sp) : "memory" )
diff --git c/include/elf.h w/include/elf.h
index 72d17c3a..7f342a23 100644
--- c/include/elf.h
+++ w/include/elf.h
enum
#define R_RISCV_TLS_DTPREL64 9
#define R_RISCV_TLS_TPREL32 10
#define R_RISCV_TLS_TPREL64 11
+#define R_RISCV_TLSDESC 12
#define R_RISCV_BRANCH 16
#define R_RISCV_JAL 17
diff --git c/src/ldso/riscv64/tlsdesc.s w/src/ldso/riscv64/tlsdesc.s
new file mode 100644
index 00000000..56d1ce89
--- /dev/null
+++ w/src/ldso/riscv64/tlsdesc.s
+.text
+.global __tlsdesc_static
+.hidden __tlsdesc_static
+.type __tlsdesc_static,%function
+__tlsdesc_static:
+ ld a0,8(a0)
+ jr t0
+
+.global __tlsdesc_dynamic
+.hidden __tlsdesc_dynamic
+.type __tlsdesc_dynamic,%function
+__tlsdesc_dynamic:
+ add sp,sp,-16
+ sd t1,(sp)
+ sd t2,8(sp)
+
+ ld t2,-8(tp) # t2=dtv
+
+ ld a0,8(a0) # a0=&{modidx,off}
+ ld t1,8(a0) # t1=off
+ ld a0,(a0) # a0=modidx
+ sll a0,a0,3 # a0=8*modidx
+
+ add a0,a0,t2 # a0=dtv+8*modidx
+ ld a0,(a0) # a0=dtv[modidx]
+ add a0,a0,t1 # a0=dtv[modidx]+off
+ sub a0,a0,tp # a0=dtv[modidx]+off-tp
+
+ ld t1,(sp)
+ ld t2,8(sp)
+ add sp,sp,16
+ jr t0
+
1 | (mkdir -p out/rv64 && cd out/rv64 && ../../configure --target=riscv64-linux-gnu && make -j 50) |
Adjust ~/musl/out/rv64/lib/musl-gcc.specs and update
~/musl/out/rv64/obj/musl-gcc 1
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4cat > ~/musl/out/rv64/obj/musl-gcc <<eof
#!/bin/sh
exec "${REALGCC:-riscv64-linux-gnu-gcc}" "$@" -specs ~/musl/out/rv64/lib/musl-gcc.specs
eof
I have also modified musl-clang (clang wrapper). Adjust
~/musl/out/rv64/obj/musl-clang to use
--target=riscv64-linux-musl. Adjust
~/musl/out/rv64/obj/ld.musl-clang to define
cc="/tmp/Rel/bin/clang --target=riscv64-linux-gnu" and
invoke exec /tmp/Rel/bin/ld.lld "$@" -lc.
Prepare a runtime test mentioned at the end of https://maskray.me/blog/2021-02-14-all-about-thread-local-storage
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42cat > ./a.c <<eof
#include <assert.h>
int foo();
int bar();
int main() {
assert(foo() == 2);
assert(foo() == 4);
assert(bar() == 2);
assert(bar() == 4);
}
eof
cat > ./b.c <<eof
#include <stdio.h>
__thread int tls0;
extern __thread int tls1;
int foo() { return ++tls0 + ++tls1; }
static __thread int tls2, tls3;
int bar() { return ++tls2 + ++tls3; }
eof
echo '__thread int tls1;' > ./c.c
sed 's/ /\t/' > ./Makefile <<'eof'
.MAKE.MODE = meta curDirOk=true
CC := ~/musl/out/rv64/obj/musl-clang -O1 -g -fpic -mtls-dialect=desc -w
LDFLAGS := -Wl,-rpath=.
all: a0 a1 a2
run: all
./a0 && ./a1 && ./a2
c.so: c.o; ${LINK.c} -shared $> -o $@
bc.so: b.o c.o; ${LINK.c} -shared $> -o $@
b.so: b.o c.so; ${LINK.c} -shared $> -o $@
a0: a.o b.o c.o; ${LINK.c} $> -o $@
a1: a.o b.so; ${LINK.c} $> -o $@
a2: a.o bc.so; ${LINK.c} $> -o $@
eof
bmake run => succeeded!
1 | % bmake run |
Test GCC
During my development of the linker patch, the Clang Driver patch was actually not ready yet. I used a more hacky approach by compiling using GCC, replacing some assembly fragments with TLSDESC code sequences, and assemblying using Clang.
Compile b.c to bb.s. Replace
general-dynamic code sequences (e.g.
la.tls.gd a0,tls0; call __tls_get_addr@plt) with TLSDESC,
e.g. 1
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6.Ltlsdesc_hi0:
auipc a0, %tlsdesc_hi(tls0)
ld a1, %tlsdesc_load_lo(.Ltlsdesc_hi0)(a0)
addi a0, a0, %tlsdesc_add_lo(.Ltlsdesc_hi0)
jalr t0, 0(a1), %tlsdesc_call(.Ltlsdesc_hi0)
add a0, a0, tp
Create an alias bin/ld.lld to be used with
-Bbin -fuse-ld=lld. I made some adjustment to the
Makefile so that an invocation looks like:
1 | % bmake run |