[PATCH] D130311: [RISCV] Enable strict FP in clang as long as Zve* or V are not enabled.

Philip Reames via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Aug 1 11:35:50 PDT 2022

reames added inline comments.

Comment at: clang/lib/Basic/Targets/RISCV.cpp:286
+  // StrictFP support for vectors is incomplete.
+  if (ISAInfo->hasExtension("zve32x"))
+    HasStrictFP = false;
craig.topper wrote:
> reames wrote:
> > craig.topper wrote:
> > > reames wrote:
> > > > asb wrote:
> > > > > There's also code in RISCVISAInfo.cpp that does `HasVector = Exts.count("zve32x") != 0`. It's probably worth adding a helper (`hasVInstructions`?) that encapsulates this, and use it from both places.
> > > > It's not clear to me why this condition is specific to embedded vector variants.  Do we have strict FP with +V?  Either you need to fix a comment here, or the condition.  One or the other.  
> > > V implies Zve64d implies Zve64f implies Zve32f and Zve64x. Zve32f implies Zve32x. Zve32x is the root of the vector inheritance tree.
> > So, I went digging.  I agree that our *implementation* treats V as implying Zve64d, but I can find anything in the *specification* to that effect.  The feature set seems like it might be identical between the two, but I don't see anything in the spec which requires a +V implementation to claim support for Zve64d.  Do you have particular wording in mind I'm missing?  
> > 
> > (Regardless, the fact we assume this elsewhere means this is a non-blocking comment for this review.  At the very least, this isn't introducing a new problem.)
> We removed the implication for a brief period but Krste and Andrew disagreed. I believe this is now covered by the note at the end of https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#183-v-vector-extension-for-application-processors
> "As is the case with other RISC-V extensions, it is valid to include overlapping extensions in the same ISA string. For example, RV64GCV and RV64GCV_Zve64f are both valid and equivalent ISA strings, as is RV64GCV_Zve64f_Zve32x_Zvl128b."
Er, yuck that's subtle.  Not quite sure I'd read it the way you do, but your read is at least easily defensible.  We can wait until someone has a concrete case where they aren't implied before figuring out if that case is disallowed per the spec.  :)

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