https://bugs.llvm.org/show_bug.cgi?id=41722
Bug ID: 41722
Summary: Poor optimization of thread local globals on OSX
Product: new-bugs
Version: 8.0
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: tyler.m.k...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
Multiple calls to tlv_get_addr are (often) generated per usage of a thread
local variable on OSX. This issue was discovered by looking at the assembly
generated by rustc, and is discussed in more detail here:
https://github.com/rust-lang/rust/pull/60341#issuecomment-487982828
I know very little about llvm - so hopefully this all makes sense. The linked
IR [1] demonstrates the issue. Often, the optimizer spits out IR which
references thread_local globals multiple times when the unoptimized IR only
references them once. Often associated with getelementptr.
In the final assembly the asm does the tlv_get_addr dance twice.
movq _foo@TLVP(%rip), %rdi
callq *(%rdi)
For larger structures with multiple members, the problem gets worse, resulting
in many redundant calls to tlv_get_addr. In contrast, when targeting linux,
__tls_get_addr@PLT, is only invoked once.
Maybe there's a good reason the address isn't cached on OSX, but I'm hoping
there isn't :).
[1]: https://gist.github.com/alexcrichton/a9a90412152d04caa7011042aa89b6bf
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