[llvm-dev] Semantics of LLVM IR intermediate variables

[Doerfert, Johannes via llvm-dev]

Once promoted, which can be expected to be the default for stack allocations, the name %x makes more sense again.
(Afaik, the alloca name is used for new instructions, e.g., PHIs. [0])
The alternative would be to rename during promotion (SROA/mem2reg) but it's unclear if that is worth it.

Cheers,
  Johannes

[0] https://godbolt.org/z/loZOfX

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of James Clarke via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Friday, October 25, 2019 18:17
To: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Semantics of LLVM IR intermediate variables

On Fri, Oct 25, 2019 at 08:23:25AM +0000, Mehdi AMINI via llvm-dev at lists.llvm.org wrote:
> On Fri, Oct 25, 2019 at 12:57 AM Jason Thomas via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> >
> > Hi all,
> >
> > This question may sound stupid, but every time I look at the IR, I take
> > some time to convince myself the following:
> >
> > The following C source code:
> >
> > 1 int x ;
> > 2 int * p ;
> > 3 p = & x ;
> >
> > when compiled to LLVM IR using clang generates the following instructions:
> >
> > 1 % x = alloca i32 , align 4
> > 2 % p = alloca i32 * , align 8
> > 3 store i32 * %x , i32 ** %p , align 8
> >
> > All the local variables in the C source code, i.e. 'x' and 'p' are
> > pointers now, in fact they are pointers with one level deeper nesting
> > level. What I mean is, 'x' is an 'int' in the C source, but '%x' is 'i32*'.
> > 'p' is 'int*' in the C source, but '%p' is 'i32**'. Doesn't it make the IR
> > naming convention a misnoer compared to their C counterpart? Shouldn't
> > '%x.addr' or '%p.addr' a better naming convention? Is there anything that I
> > am missing?
> >
>
> You're right that the name isn't the most accurate from a clang point of
> view, redirecting to cfe-dev@ if anyone has an opinion from the clang side.

This is, however, consistent with global variables, where @foo is &foo
in the C source, which also matches what the linker's view is (where the
value of a symbol is its address).

James
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