(Yet another) Optimization for TLS access #17220
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yuyichao
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Also switch to using |
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| // more efficient since we can have better control over the creation and | ||
| // initialization of the TLS buffer. | ||
| // | ||
| // On platforms that use ELF (Linux, FreeBSD), we use a `__thread` variable. |
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* Use indirect function (`ifunc`) to set the address of a symbol at runtime. * Use `weak` symbol to detect (in `ifunc`) whether the static version of TLS is available.
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This time for C code on Linux. (
I've probably hit a linker bug on aarch64.....Found workaround)ifunc) to set the address of a symbol at runtime.weaksymbol to detect (inifunc) whether the static version ofTLS is available.
This is actually pretty much the original plan for #14083 (i.e. resolve the symbol to the better version at linker level) before realizing that undefined symbol in shared library is not really well supported on all platforms, and weak symbol doesn't do what most people (me included) would naively assume ;-p....
The use of
ifuncin this PR is not particularly reliably in that the weak symbol check might return missing of the static version when it actually exist if the linker does the relocation in the wrong order (ld tries to do it in the right order though and I haven't seen any issue like that on the platforms I've tested). In such case, this should just fallback to the same code with the one on current master.This is the last bit of optimization I can imagine for linux (apart from starting to pass it around more). The actual performance gain is a hard to measure.