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@@ -17,8 +17,19 @@ when ODIN_OS == "windows" {
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// necessarily export a symbol named setjmp but rather _setjmp in the case
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// of musl, glibc, BSD libc, and msvcrt.
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//
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- // TODO(mv): Some description of the extra argument, and maybe a link describing
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- // it in more detail?
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+ /// NOTE(dweiler): UCRT has two implementations of longjmp. One that performs
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+ // stack unwinding and one that doesn't. The choice of which to use depends on a
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+ // flag which is set inside the jmp_buf structure given to setjmp. The default
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+ // behavior is to unwind the stack. Within Odin, we cannot use the stack
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+ // unwinding version as the unwinding information isn't present. To opt-in to
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+ // the regular non-unwinding version we need a way to set this flag. Since the
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+ // location of the flag within the struct is not defined or part of the ABI and
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+ // can change between versions of UCRT, we must rely on setjmp to set it. It
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+ // turns out that setjmp receives this flag in the RDX register on Win64, this
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+ // just so happens to coincide with the second argument of a function in the
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+ // Win64 ABI. By giving our setjmp a second argument with the value of zero,
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+ // the RDX register will contain zero and correctly set the flag to disable
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+ // stack unwinding.
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@(link_name="_setjmp")
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setjmp :: proc(env: ^jmp_buf, hack: rawptr = nil) -> int ---;
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}
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vassvik