[LLVMdev] ideas for TLS implementation

[Lauro Ramos Venancio]

For everyone understand which code must be emitted to implement TLS, I
will paste the code generated by gcc for a simple function:

__thread int a = 1;

int f(){
  return a;
}

gcc teste.c -o teste.s -S -O2 (arm-linux-gnueabi):


        .global a
        .section        .tdata,"awT",%progbits <== special section for
tls symbols
        .align  2
        .type   a, %object
        .size   a, 4
a:
        .word   1
        .text
        .align  2
        .global f
        .type   f, %function
f:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        str     lr, [sp, #-4]!
        ldr     r3, .L2
        bl      __aeabi_read_tp @ load_tp_soft <== get thread pointer
(abi dependent)
        ldr     r0, [r0, r3]
        ldr     pc, [sp], #4
.L3:
        .align  2
.L2:
        .word   a(tpoff)   <== offset of "a"
        .size   f, .-f


The idea of implementing TLS is to create the "thread_local" keyword.
The program above would be converted to this:

@a = thread_local global i32 1          ; <i32*> [#uses=1]  <== Thread
local keyword

define i32 @f() {
entry:
        %tmp1 = load i32* @a            ; <i32> [#uses=1]
        ret i32 %tmp1
}

With "thread_local" keyword, the backends can emit TLS code. The TLS
is architecture and abi dependent, so I think we must let the backends
handle with TLS symbols. Each backend must handle each TLS model.

> Adding TLS support seems to be tricky, since there are 4
> different TLS "models" with different "assumptions".

There is a model ("general dynamic") that works in all situations. The
other three models are optimizations of the first. So, I think we
should start implementing the "general dynamic" model.

I believe the roadmap is:

1. make LLVM understand "thread_local" keyword. (AsmParser, AsmWriter,
BCReader, BCWriter)
2. make llvm-gcc emit "thread_local" keyword
3. implement "thread_local" in C backend (model independent)
3. implement "general dynamic" tls model for i386-linux-gnu
4. implement "general dynamic" tls model for arm-linux-gnueabi (this
is more complex because gcc 4.0 doesn't support tls for arm. I will
have to backport a patch from gcc 4.1 to llvm-gcc)
5. Maybe implement other models for arm-linux-gnueabi

Suggestions are appreciated. :-)

Lauro