RE: [ARM] mno-pic-data-is-text-relative & msingle-pic-base

["Joey Ye" <joey dot ye at foss dot arm dot com>]

Nathan,

This patch will do what you intend it to do. However, I am not sure in part related to VxWorks. The logic behind this patch is that -mno-pic-data-is-text-relative should enable -msingle-pic-base because otherwise it will be useless. The logic itself is orthogonal to OS. So I am not convinced the 'else if' shouldn't be just 'if'. It should not change VxWorks behaviour if VxWorks enables -msingle-pic-base explicitly. Or otherwise there is at least one use case that -mno-pic-data-is-text-relative can be used without -msingle-pic-base, which breaks the logic that this whole patch stands on.

Thanks,
Joey

> -----Original Message-----
> From: Nathan Sidwell [mailto:nathanmsidwell@gmail.com] On Behalf Of
> Nathan Sidwell
> Sent: 09 May 2016 15:07
> To: Richard Earnshaw; GCC Patches
> Cc: Joey Ye
> Subject: [ARM] mno-pic-data-is-text-relative & msingle-pic-base
> 
> This patch comes from an off-list conversation between Joey & me.  The
> context is with RTOSs not all singing & dancing dynamic objects and OSes.
> 
> currently, the documentation for -mno-pic-data-is-text-relative (-mno-PDITR)
> says 'Assume that each data segments are relative to text segment at load
> time.
>   Therefore, it permits addressing data using PC-relative operations.
>   This option is on by default for targets other than VxWorks RTP.'
> 
> However, if you use just this option, you still end up with a pic-register init
> sequence that  presumes a fixed mapping.  That's a surprise.  Joey tells me
> its expected use is with -msingle-pic-base (-mSPB), which reserves a global
> register to point at the (single) GOT.  That's what I had expected the -mno-
> PDITR option to have implied.
> 
> Apparently there are legitimate reasons one might want the -mno-PDITR
> behaviour without -mSPB.  I don't know what those are, perhaps Joey could
> clarify?
> 
> Anyway, IMHO that is the rare case and the more common case is that one
> would want to have -mnoPDITR imply -mSPB. (The reverse probably doesn't
> apply.)
> 
> This patch does 3 things.
> 1) have -mno-PDITR imply -mSPB, unless one has explictly provided -m[no-
> ]SPB.
> 2) clarified  the -m[no-]PDITR documentation.
> 3) Added some testcases -- there didn't appear to be any.
> 
> ok?
> 
> nathan