[RISCV][MC] Warn if SEW/LMUL may not be compatible

According to RVV spec:
> 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.
>
> For a given supported fractional LMUL setting, implementations
> must support SEW settings between SEWMIN and LMUL * ELEN, inclusive.

We print a warning if these requirements are not met.

Reviewers: kito-cheng, asb, frasercrmck, jrtc27, michaelmaitland, lukel97

Reviewed By: lukel97

Pull Request: #94313

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

@@ -2155,6 +2155,16 @@ bool RISCVAsmParser::parseVTypeToken(const AsmToken &Tok, VTypeState &State,
       break;
     if (!RISCVVType::isValidLMUL(Lmul, Fractional))
       break;
+
+    if (Fractional) {
+      unsigned ELEN = STI->hasFeature(RISCV::FeatureStdExtZve64x) ? 64 : 32;
+      unsigned MinLMUL = ELEN / 8;
+      if (Lmul > MinLMUL)
+        Warning(Tok.getLoc(),
+                "use of vtype encodings with LMUL < SEWMIN/ELEN == mf" +
+                    Twine(MinLMUL) + " is reserved");
+    }
+
     State = VTypeState_TailPolicy;
     return false;
   }
@@ -2194,6 +2204,7 @@ ParseStatus RISCVAsmParser::parseVTypeI(OperandVector &Operands) {
   bool MaskAgnostic = false;
 
   VTypeState State = VTypeState_SEW;
+  SMLoc SEWLoc = S;
 
   if (parseVTypeToken(getTok(), State, Sew, Lmul, Fractional, TailAgnostic,
                       MaskAgnostic))
@@ -2211,6 +2222,16 @@ ParseStatus RISCVAsmParser::parseVTypeI(OperandVector &Operands) {
 
   if (getLexer().is(AsmToken::EndOfStatement) && State == VTypeState_Done) {
     RISCVII::VLMUL VLMUL = RISCVVType::encodeLMUL(Lmul, Fractional);
+    if (Fractional) {
+      unsigned ELEN = STI->hasFeature(RISCV::FeatureStdExtZve64x) ? 64 : 32;
+      unsigned MaxSEW = ELEN / Lmul;
+      // If MaxSEW < 8, we should have printed warning about reserved LMUL.
+      if (MaxSEW >= 8 && Sew > MaxSEW)
+        Warning(SEWLoc,
+                "use of vtype encodings with SEW > " + Twine(MaxSEW) +
+                    " and LMUL == mf" + Twine(Lmul) +
+                    " may not be compatible with all RVV implementations");
+    }
 
     unsigned VTypeI =
         RISCVVType::encodeVTYPE(VLMUL, Sew, TailAgnostic, MaskAgnostic);

llvm/test/MC/RISCV/rvv/vsetvl.s

@@ -1,5 +1,7 @@
 # RUN: llvm-mc -triple=riscv64 -show-encoding --mattr=+v %s \
 # RUN:        | FileCheck %s --check-prefixes=CHECK-ENCODING,CHECK-INST
+# RUN: llvm-mc -triple=riscv64 -show-encoding --mattr=+zve32x %s 2>&1 \
+# RUN:        | FileCheck %s --check-prefix=CHECK-ZVE32X
 # RUN: not llvm-mc -triple=riscv64 -show-encoding %s 2>&1 \
 # RUN:        | FileCheck %s --check-prefix=CHECK-ERROR
 # RUN: llvm-mc -triple=riscv64 -filetype=obj --mattr=+v %s \
@@ -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-ZVE32X: :[[#@LINE-2]]:17: warning: use of vtype encodings with SEW > 16 and LMUL == mf2 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-ZVE32X: :[[#@LINE-2]]:17: warning: use of vtype encodings with SEW > 8 and LMUL == mf4 may not be compatible with all RVV implementations{{$}}
 # CHECK-ENCODING: [0x57,0x76,0x65,0x0d]
 # CHECK-ERROR: instruction requires the following: 'V' (Vector Extension for Application Processors), 'Zve32x' (Vector Extensions for Embedded Processors){{$}}
 # CHECK-UNKNOWN: 0d657657 <unknown>
 
 vsetvli a2, a0, e32, mf8, ta, ma
 # CHECK-INST: vsetvli a2, a0, e32, mf8, ta, ma
+# CHECK-ZVE32X: :[[#@LINE-2]]:22: warning: use of vtype encodings with LMUL < SEWMIN/ELEN == mf4 is reserved{{$}}
 # CHECK-ENCODING: [0x57,0x76,0x55,0x0d]
 # CHECK-ERROR: instruction requires the following: 'V' (Vector Extension for Application Processors), 'Zve32x' (Vector Extensions for Embedded Processors){{$}}
 # CHECK-UNKNOWN: 0d557657 <unknown>