[LLVMdev] _Znwm is not a builtin
bob.wilson at apple.com
Wed May 15 21:42:04 PDT 2013
On May 15, 2013, at 9:10 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Wed, May 15, 2013 at 8:57 PM, Chris Lattner <clattner at apple.com> wrote:
> On May 15, 2013, at 8:50 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>> 1) The 'nobuiltin' attribute doesn't actually prevent the optimization (see recent patch on llvmcommits)
>>> 2) We can't block the optimization if the call happens through a function pointer, unless we also annotate all calls through function pointers as 'nobuiltin'
>>> How feasible would it be to make the 'builtin-ness' of _Znwm etc be opt-in rather than opt-out? Is there some other option we could pursue?
>>> Wow, this was spectacularly unclear, sorry about that. To avoid confusion, I'm suggesting that we add a 'builtin' attribute, and do not treat a call to _Znwm as a builtin call unless it has the attribute.
>> It's not clear to me that "builtin" is the right way to model this, but it definitely sounds like this should be an attribute on a call site (as opposed to on the function itself). What specific kinds of optimizations are we interested in doing to _Znwm calls?
>> Initially, I'm just concerned about keeping the optimizations we already perform, such as globalopt lowering a new/delete pair into a global, while disabling the non-conforming variations of those optimizations. But we're also permitted to merge multiple allocations into one if they have sufficiently similar lifetimes.
> So your proposal is for Clang to slap the attribute on explicit calls to ::operator new, but any other use of the symbol (e.g. from C code or something else weird) can be optimized?
> No, because Clang cannot statically detect which indirect calls might call ::operator new. Instead, my proposal is to add a 'builtin' attribute to LLVM, and then for clang to add that attribute to the calls which can be optimized.
> If you think the C code / weird cases are important, a more nuanced option springs to mind:
> * Allow the 'nobuiltin' attribute on function declarations
> * Add a 'builtin' attribute, permitted only on direct calls to 'nobuiltin' functions, which overrides the 'nobuiltin' attribute on the function
> Would that be preferable?
Just curious: will this fix the issue that we discussed last month in the context of r179071 regarding the need to compile with -fno-builtin if you have code with custom new operators? (http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130408/170888.html)
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