[llvm-branch-commits] [RISCV][MC] Warn if SEW/LMUL may not be compatible (PR #94313)

[Craig Topper via llvm-branch-commits]

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@@ -71,18 +73,21 @@ vsetvli a2, a0, e32, m8, ta, ma
 
 vsetvli a2, a0, e32, mf2, ta, ma
 # CHECK-INST: vsetvli a2, a0, e32, mf2, ta, ma
+# CHECK-WARNING: :[[#@LINE-2]]:17: warning: SEW > 16 may not be compatible with all RVV implementations{{$}}
 # CHECK-ENCODING: [0x57,0x76,0x75,0x0d]
 # CHECK-ERROR: instruction requires the following: 'V' (Vector Extension for Application Processors), 'Zve32x' (Vector Extensions for Embedded Processors){{$}}
 # CHECK-UNKNOWN: 0d757657 <unknown>
 
 vsetvli a2, a0, e32, mf4, ta, ma
 # CHECK-INST: vsetvli a2, a0, e32, mf4, ta, ma
+# CHECK-WARNING: :[[#@LINE-2]]:17: warning: SEW > 8 may not be compatible with all RVV implementations{{$}}
----------------
topperc wrote:

> Ok, that seems reasonable. Should we maybe then reword the LMUL < SEWMIN/ELEN case to mention that the encoding is actually reserved, whereas for SEW > LMUL * ELEN it may just not be compatible

I don't think it is reserved. This is the paragraph from the spec

```
Implementations must provide fractional LMUL settings that allow the narrowest supported type to occupy a fraction of a vector register corresponding to the ratio of the narrowest supported type’s width to that of the largest supported type’s width. In general, the requirement is to support LMUL ≥ SEWMIN/ELEN, where SEWMIN is the narrowest supported SEW value and ELEN is the widest supported SEW value. In the standard extensions, SEWMIN=8. For standard vector extensions with ELEN=32, fractional LMULs of 1/2 and 1/4 must be supported. For standard vector extensions with ELEN=64, fractional LMULs of 1/2, 1/4, and 1/8 must be supported.
```

It doesn't say it can't be supported. It only says what must be supported.

https://github.com/llvm/llvm-project/pull/94313