[PATCH] D113294: [IR] Remove unbounded as possible value for vscale_range minimum

[Paul Walker via Phabricator via llvm-commits]

paulwalker-arm added inline comments.


================
Comment at: clang/lib/Basic/Targets/AArch64.cpp:476-484
+  assert(LangOpts.VScaleMin && "vscale min must be greater than 0!");
+
+  if (LangOpts.VScaleMax)
     return std::pair<unsigned, unsigned>(LangOpts.VScaleMin,
                                          LangOpts.VScaleMax);
+
   if (hasFeature("sve"))
----------------
c-rhodes wrote:
> paulwalker-arm wrote:
> > This looks like a change of behaviour to me.  Previously the command line flags would override the "sve" default but now that only happens when the user specifies a maximum value.  That means the interface can no longer be used to force truly width agnostic values.
> > This looks like a change of behaviour to me.  Previously the command line flags would override the "sve" default but now that only happens when the user specifies a maximum value.  That means the interface can no longer be used to force truly width agnostic values.
> 
> I think the issue here is the default of 1 for min would always trigger `if (LangOpts.VScaleMin || LangOpts.VScaleMax)` overriding the SVE default. Perhaps the default can be removed from the driver option and handled here, i.e.
> 
> ```
> if (LangOpts.VScaleMin || LangOpts.VScaleMax)
>     return std::pair<unsigned, unsigned>(LangOpts.VScaleMin ? LangOpts.VScaleMin : 1,
>                                          LangOpts.VScaleMax);
> ```
> 
> 
Is this enough?  I'm not sure it'll work because `LangOpts.VScaleMin` defaults to 1 and thus you'll always end up passing the first check, unless the user specifically uses `-mvscale-min=0` which they cannot because that'll result in `diag::err_cc1_unbounded_vscale_min`.

Do we need to link the LangOpt defaults to the attribute defaults?  I'm think that having the LangOpts default to zero is a good way to represent "value is unspecified" with regards to the `LangOpts.VScaleMin`.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D113294/new/

https://reviews.llvm.org/D113294