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DwarfInstructions.hpp

Differential D46971

[DWARF] Get RA from RA register even if it appears unused
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Authored by whitequark on May 16 2018, 12:17 PM.

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compnerd

Summary

If prolog info lists the RA register as unused, it means that
the current stack frame corresponds to a leaf function which never
needs to save the RA register. The RA register is of course used
to return from this leaf function.

Such a frame may be in the middle of the stack if another frame was
pushed on top of it by a signal or CPU exception. In this case,
unless we extract RA from the RA register, _Unwind_Backtrace would
stop at the frame right above the signal or exception frame.

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whitequark created this revision.May 16 2018, 12:17 PM

Herald added a subscriber: JDevlieghere. · View Herald TranscriptMay 16 2018, 12:17 PM

Reuploaded diff with context.

@compnerd ping

It would be nice if we had a test case added for this, but, seems correct to me.

src/DwarfInstructions.hpp
195	I think that we should stick to LLVM style (coddled braces) and use C++ style casts.

It would be nice if we had a test case added for this

This is inherently architecture and platform specific, and libunwind currently doesn't have any provisions for this... In principle, would a x86-only Linux-only test suffice? I'm not sure if I can spend time making it more portable.

src/DwarfInstructions.hpp
195	Ack re braces. I was copying that cast from ten lines above. Should I change that too? This should be a static_cast, right?

x86-Linux-only test is good. LG with the discussed changes.

src/DwarfInstructions.hpp
195	Yeah, it should be a `static_cast`. And, yes, it should be fine to adjust the style there too (just do it as a separate commit).

This revision is now accepted and ready to land.Jun 12 2018, 8:05 PM

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DwarfInstructions.hpp

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Diff 147152

src/DwarfInstructions.hpp

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	else	else
	return UNW_EBADREG;	return UNW_EBADREG;
	}	}
		else if (i == (int)cieInfo.returnAddressRegister)
		compnerdUnsubmitted Not Done Reply Inline Actions I think that we should stick to LLVM style (coddled braces) and use C++ style casts. compnerd: I think that we should stick to LLVM style (coddled braces) and use C++ style casts.
		whitequarkAuthorUnsubmitted Not Done Reply Inline Actions Ack re braces. I was copying that cast from ten lines above. Should I change that too? This should be a static_cast, right? whitequark: Ack re braces. I was copying that cast from ten lines above. Should I change that too? This…
		compnerdUnsubmitted Not Done Reply Inline Actions Yeah, it should be a `static_cast`. And, yes, it should be fine to adjust the style there too (just do it as a separate commit). compnerd: Yeah, it should be a `static_cast`. And, yes, it should be fine to adjust the style there too…
		returnAddress = registers.getRegister(i);
	}	}

	// By definition, the CFA is the stack pointer at the call site, so	// By definition, the CFA is the stack pointer at the call site, so
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