[llvm-dev] maximum value for alignstack function attribute?

[Ben Craig via llvm-dev]

There could be performance benefits to aligning to larger sizes.  Some memory controllers really like it when you start at a page boundary for example.

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Friedman, Eli via llvm-dev
> Sent: Monday, October 23, 2017 4:59 PM
> To: Andrew Kelley <superjoe30 at gmail.com>; LLVM Dev <llvm-
> dev at lists.llvm.org>
> Subject: Re: [llvm-dev] maximum value for alignstack function attribute?
> 
> On 10/23/2017 1:02 PM, Andrew Kelley via llvm-dev wrote:
> > I have this test case in my frontend, which is triggering an assert in
> > llvm master branch:
> >
> > fn fnWithAlignedStack() -> i32 {
> >     @setAlignStack(1024);
> >     return 1234;
> > }
> >
> > It sets alignstack=1024 in the function attributes (see
> > https://urldefense.proofpoint.com/v2/url?u=http-
> 3A__llvm.org_docs_Lang
> > Ref.html-23function-
> 2Dattributes&d=DwIGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA
> &r=y8mub81SfUi-
> UCZRX0Vl1g&m=0DdfVtZNBtfRLf7sV4nDkOZcRe2iDuboLEtD2AwEtAA&s=Tfr
> Mg902OEsdzbClQaj1_afFH3xp9lwmeSNyJELBGCw&e=). It's tripping an
> assert:
> >
> > llvm/lib/IR/Attributes.cpp:134: static llvm::Attribute
> > llvm::Attribute::getWithStackAlignment(llvm::LLVMContext&, uint64_t):
> > Assertion `Align <= 0x100 && "Alignment too large."'
> >
> > Is 0x100 the correct maximum alignstack value? Can I submit a patch to
> > document this in LangRef.html?
> 
> Yes...?  But nobody should ever specify a value anywhere near that large.
> 
> The point of the attribute is to fixup the stack alignment in cases of ABI
> mismatches, so you normally don't want to specify a value higher than the
> actual ABI alignment of the stack.  If you want to allocate aligned memory on
> the stack, you can just set the alignment of an alloca, and the code generator
> will provide memory with the appropriate alignment.
> 
> -Eli
> 
> --
> Employee of Qualcomm Innovation Center, Inc.
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