[PATCH] Allocate stack storage for .block_descriptor and captured self.

[Adrian Prantl]

On Feb 27, 2013, at 3:17 PM, Eric Christopher <echristo at gmail.com> wrote:

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> On Wed, Feb 27, 2013 at 11:49 AM, John McCall <rjmccall at apple.com> wrote:
> On Feb 27, 2013, at 11:42 AM, Eric Christopher <echristo at gmail.com> wrote:
>> On Wed, Feb 27, 2013 at 11:39 AM, Adrian Prantl <aprantl at apple.com> wrote:
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>> On Feb 27, 2013, at 11:31 AM, John McCall <rjmccall at apple.com> wrote:
>> > Okay, you're saying that the value is actually no longer live at all at this point in the function?  It seems reasonable to lose track of the debug info then, although we should be leaving behind a marker in the DWARF that says the value is unavailable.
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>> > If we want to make stronger guarantees in -O0 for purposes of debugging — and I think that's reasonable — then throwing the value in an alloca is acceptable.
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>> To clarify: Are you suggesting to only generate the alloca at -O0, or are you comfortable with it as it is?
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>> If the value isn't live past that spot I'm more comfortable with dropping the debug info there rather than changing the generated code to keep the value live through the end of the function.
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> Purely out of attachment to the principle that debug info shouldn't change the code?
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> Pretty much.
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> Not losing information has intrinsic value in a debug build.  If we need to emit slightly different code in order to force a value to stay live and thus improve the debugging experience, then so be it.
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> Agreed that making the experience better is desirable, but it can make debugging a problem more difficult if the code changes when you turn on debugging symbols.

I agree that turning on -g should not yield different code, but this is not what the patch does. Right now it changes code generation for all optimization levels. In the sake of efficiency we could emit the extra alloca at -O0 only. Would that work for you?

— adrian