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libgo patch committed: Add precise stack scan support

This libgo patch by Cherry Zhang adds support for precise stack
scanning to the Go runtime.  This uses per-function stack maps stored
in the exception tables in the language-specific data area.  The
compiler needs to generate these stack maps; currently this is only
done by a version of LLVM, not by GCC.  Each safepoint in a function
is associated with a (real or dummy) landing pad, and its "type info"
in the exception table is a pointer to the stack map. When a stack is
scanned, the stack map is found by the stack unwinding code.

For precise stack scan we need to unwind the stack. There are three cases:

- If a goroutine is scanning its own stack, it can unwind the stack
and scan the frames.

- If a goroutine is scanning another, stopped, goroutine, it cannot
directly unwind the target stack. We handle this by switching
(runtime.gogo) to the target g, letting it unwind and scan the stack,
and switch back.

- If we are scanning a goroutine that is blocked in a syscall, we send
a signal to the target goroutine's thread, and let the signal handler
unwind and scan the stack. Extra care is needed as this races with
enter/exit syscall.

Currently this is only implemented on GNU/Linux.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.


Attachment: patch.txt
Description: Text document

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