Miscompilation: Missing prefixes for emulatedTLS variables on macOS/ARM64 with -O0

[sschaller]

Reproducer:
https://alive2.llvm.org/ce/z/6Xr9_c

struct T {
    T() {} // required
    int a = 0;
};

thread_local T aa;

int main() {
    return 0;
}

Trying to compile this code on macOS / ARM64 with clang -stdlib=libc++ -std=c++17 -femulated-tls main.cpp
leads to a linker error for undefined symbols ___tls_guard and _aa (4-5, 60-61 in the reproducer).

Inspecting the generated assembly code shows that both symbols are used as operands instead of the prefixed variables ___emutls_v.__tls_guard and ___emutls_v.aa.

Compiling with -O1 correctly prefixes the operands and fixes the issue.

I'm not quite sure what is missing and would appreciate any pointers.

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Sebastian Schaller (sschaller)

Reproducer: https://alive2.llvm.org/ce/z/6Xr9_c

struct T {
    T() {} // required
    int a = 0;
};

thread_local T aa;

int main() {
    return 0;
}

Trying to compile this code on macOS / ARM64 with clang -stdlib=libc++ -std=c++17 -femulated-tls main.cpp
leads to a linker error for undefined symbols ___tls_guard and _aa (4-5, 60-61 in the reproducer).

Inspecting the generated assembly code shows that both symbols are used as operands instead of the prefixed variables ___emutls_v.__tls_guard and ___emutls_v.aa.

Compiling with -O1 correctly prefixes the operands and fixes the issue.

I'm not quite sure what is missing and would appreciate any pointers.

[davemgreen]

It sounds like it is not implemented for GlobalISel (GISel is used at -O0, SDAG for higher optimization levels). See 01eb869 for where that was implemented, vs AArch64TargetLowering::LowerGlobalTLSAddress that has the option to use TargetLowering::LowerToTLSEmulatedModel.

[sschaller]

Hi @davemgreen I just wanted to say thank you for pinpointing the issue. I probably wouldn't have found it by myself.
I created a PR with a fix that will fall back to SDAG in this case. I shortly tried to implement it, but quickly found myself out of depth.