[LLVMdev] _Znwm is not a builtin

[Michael Gottesman]

Just as an FYI: I am going to push this patch forward. I need this in the near term and to quote the great zygoloid:

<zygoloid> gottesmm: if you want it done soon, feel free to take over that                                   
           patch and run with it
<zygoloid> gottesmm: (otherwise i'll circle back to it in the great blue                                     
           eventually...)

Michael

On May 20, 2013, at 8:03 AM, Chris Lattner <clattner at apple.com> wrote:

> On May 16, 2013, at 12:36 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> 
>> 
>> Since it would probably help to quantify the complexity increase, I've implemented my more recent suggestion (patch attached). This patch allows 'nobuiltin' on a function declaration or definition, and adds a 'builtin' attribute which can only be present on a call site for a direct call to a function declared with the 'nobuiltin' attribute. The 'builtin' attribute has the effect of canceling out the 'nobuiltin' attribute on the declaration.
> 
> Ok.  Please make "hasFnAttr(Attribute::NoBuiltin)" abort though: it will be a common bug to call this instead of your new accessor.
> 
>> This (incidentally) also exactly matches what we want for another Clang feature: we want a -fno-builtin-foo which makes 'foo' not be a builtin (but we still want __builtin_foo to have the builtin behavior). This, again, is not possible with the existing 'nobuiltin' attribute, due to the function pointer problem. Instead, Clang currently just provides -fno-builtin, and even *that* only provides a broken half-implementation -- it calls TargetLibraryInfo::disableAllFunctions, whose effect is not preserved across "clang -fno-builtin -emit-llvm | opt", nor across LTO.
> 
> There are several problems in this space.  While your new attribute may be useful to help in this area, it is definitely not sufficient to solve the problem.  -fno-builtin-strlen should disable transformations from strchr->strlen (as a random example) even if there is no LLVM IR function for strlen.
> 
> -Chris
> 
> 
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