[PATCH] Warn on use of vector initializers in ARM BE mode

[James Molloy]

Hi Alp,

I certainly thought about this, however it's not exactly clear how to
generate good fixits. I can generate fixits that "kind of work", but my
understanding was that the rule of thumb is that we shouldn't generate a
fixit unless we're pretty sure it's correct.

The main problem is that we must create a fixit on all the usage sites. The
fix would be:

  1. Convert the vector declaration into an array declaration.
  2. Add vld* calls on each use of the declaration.

Which is fairly difficult in itself (updating all uses of a decl is more
difficult than just adding a warning on the decl itself), but also to
maintain the semantics of the program we'd need to add vst* calls (store the
result back) whenever the declaration may be modified either by arithmetic
or an intrinsic.

Because of this complexity, I felt it wasn't really worth the investment to
generate proper fixits.

Cheers,

James

> -----Original Message-----
> From: Alp Toker [mailto:alp at nuanti.com]
> Sent: 19 June 2014 09:44
> To: James Molloy; Clang Commits
> Subject: Re: [PATCH] Warn on use of vector initializers in ARM BE mode
> 
> 
> On 19/06/2014 10:58, James Molloy wrote:
> >
> > Hi,
> >
> > The ability to use vector initializer lists is a GNU vector extension
> > and is unrelated to the NEON intrinsics in arm_neon.h. On little
> > endian machines it works fine, however on big endian machines it
> > exhibits surprising behaviour:
> >
> > uint32x2_t x = {42, 64};
> >
> > return vget_lane_u32(x, 0); // Will return 64.
> >
> > Because of this, explicitly call out that it is unsupported on big
> > endian machines.
> >
> > This patch will emit the following warning in big-endian mode:
> >
> > test.c:3:15: warning: Vector initializers are part of the GNU vector
> > extensions and are not compatible with NEON intrinsics on big endian
> > ARM targets [-Wgnu]
> >
> > int32x4_t x = {0, 1, 2, 3};
> >
> > ^
> >
> > test.c:3:15: note: Consider using vld1q_s32() to initialize a vector
> > from memory, or vcombine_s32(vcreate_s32(), vcreate_s32()) to
> > initialize from integer constants
> >
> > 1 warning generated.
> >
> > It's fairly simple, is it OK to commit?
> >
> 
> Hi James,
> 
> Would it be a big step to generate FixIts instead of describing the
> possible fixes in words?
> 
> I think as a rule of thumb, if it's worth describing code changes in
> diagnostics, it's worth attempting a FixIt.
> 
> Alp.
> 
> 
> > Cheers,
> >
> > James
> >
> >
> >
> > _______________________________________________
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> > cfe-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> 
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