[RISCV][VLS] Support RISCV VLS calling convention

This patch adds a function attribute `riscv_vls_cc` for RISCV VLS calling
convention which takes 0 or 1 argument, the argument is the `ABI_VLEN`
which is the `VLEN` for passing the fixed-vector arguments, it wraps the
argument as a scalable vector(VLA) using the `ABI_VLEN` and uses the
corresponding mechanism to handle it. The range of `ABI_VLEN` is [32, 65536],
if not specified, the default value is 128.

An option `-mriscv-abi-vlen=N` is also added to specify the `ABI_VLEN`
globally, it's used for every functions are being compiled, however if
both function attribute and option are specified, the function attribute
has higher priority than the option which means the function attribute
overwrites the `ABI_VLEN` specified by the option.

Here is an example of VLS argument passing:
Non-VLS call:
```
  void original_call(__attribute__((vector_size(16))) int arg) {}
=>
  define void @original_call(i128 noundef %arg) {
  entry:
    ...
    ret void
  }
```
VLS call:
```
  void __attribute__((riscv_vls_cc(256))) vls_call(__attribute__((vector_size(16))) int arg) {}
=>
  define riscv_vls_cc void @vls_call(<vscale x 1 x i32> %arg) {
  entry:
    ...
    ret void
  }
}
```

The first Non-VLS call passes generic vector argument of 16 bytes by
flattened integer.
On the contrary, the VLS call uses `ABI_VLEN=256` which wraps the
vector to <vscale x 1 x i32> where the number of scalable vector elements
is calaulated by: `ORIG_ELTS * RVV_BITS_PER_BLOCK / ABI_VLEN`.
Note: ORIG_ELTS = Vector Size / Type Size = 128 / 32 = 4.

Author: 4vtomat

clang/include/clang-c/Index.h

@@ -3005,6 +3005,7 @@ enum CXCallingConv {
   CXCallingConv_M68kRTD = 19,
   CXCallingConv_PreserveNone = 20,
   CXCallingConv_RISCVVectorCall = 21,
+  CXCallingConv_RISCVVLSCall = 22,
 
   CXCallingConv_Invalid = 100,
   CXCallingConv_Unexposed = 200

clang/include/clang/AST/Type.h

@@ -1942,7 +1942,7 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     /// Extra information which affects how the function is called, like
     /// regparm and the calling convention.
     LLVM_PREFERRED_TYPE(CallingConv)
-    unsigned ExtInfo : 13;
+    unsigned ExtInfo : 17;
 
     /// The ref-qualifier associated with a \c FunctionProtoType.
     ///
@@ -4395,6 +4395,8 @@ class FunctionType : public Type {
 
     // |  CC  |noreturn|produces|nocallersavedregs|regparm|nocfcheck|cmsenscall|
     // |0 .. 4|   5    |    6   |       7         |8 .. 10|    11   |    12    |
+    // |RISCV-ABI-VLEN|
+    // |13    ..    17|
     //
     // regparm is either 0 (no regparm attribute) or the regparm value+1.
     enum { CallConvMask = 0x1F };
@@ -4407,23 +4409,25 @@ class FunctionType : public Type {
     };
     enum { NoCfCheckMask = 0x800 };
     enum { CmseNSCallMask = 0x1000 };
-    uint16_t Bits = CC_C;
+    enum { Log2RISCVABIVLenMask = 0x1E000, Log2RISCVABIVLenOffset = 13 };
+    uint32_t Bits = CC_C;
 
-    ExtInfo(unsigned Bits) : Bits(static_cast<uint16_t>(Bits)) {}
+    ExtInfo(unsigned Bits) : Bits(static_cast<uint32_t>(Bits)) {}
 
   public:
     // Constructor with no defaults. Use this when you know that you
     // have all the elements (when reading an AST file for example).
     ExtInfo(bool noReturn, bool hasRegParm, unsigned regParm, CallingConv cc,
             bool producesResult, bool noCallerSavedRegs, bool NoCfCheck,
-            bool cmseNSCall) {
+            bool cmseNSCall, unsigned Log2RISCVABIVLen) {
       assert((!hasRegParm || regParm < 7) && "Invalid regparm value");
       Bits = ((unsigned)cc) | (noReturn ? NoReturnMask : 0) |
              (producesResult ? ProducesResultMask : 0) |
              (noCallerSavedRegs ? NoCallerSavedRegsMask : 0) |
              (hasRegParm ? ((regParm + 1) << RegParmOffset) : 0) |
              (NoCfCheck ? NoCfCheckMask : 0) |
-             (cmseNSCall ? CmseNSCallMask : 0);
+             (cmseNSCall ? CmseNSCallMask : 0) |
+             (Log2RISCVABIVLen << Log2RISCVABIVLenOffset);
     }
 
     // Constructor with all defaults. Use when for example creating a
@@ -4450,6 +4454,10 @@ class FunctionType : public Type {
 
     CallingConv getCC() const { return CallingConv(Bits & CallConvMask); }
 
+    unsigned getLog2RISCVABIVLen() const {
+      return (Bits & Log2RISCVABIVLenMask) >> Log2RISCVABIVLenOffset;
+    }
+
     bool operator==(ExtInfo Other) const {
       return Bits == Other.Bits;
     }
@@ -4505,6 +4513,11 @@ class FunctionType : public Type {
       return ExtInfo((Bits & ~CallConvMask) | (unsigned) cc);
     }
 
+    ExtInfo withLog2RISCVABIVLen(unsigned Log2RISCVABIVLen) const {
+      return ExtInfo((Bits & ~Log2RISCVABIVLenMask) |
+                     (Log2RISCVABIVLen << Log2RISCVABIVLenOffset));
+    }
+
     void Profile(llvm::FoldingSetNodeID &ID) const {
       ID.AddInteger(Bits);
     }
@@ -4609,6 +4622,9 @@ class FunctionType : public Type {
 
   bool getCmseNSCallAttr() const { return getExtInfo().getCmseNSCall(); }
   CallingConv getCallConv() const { return getExtInfo().getCC(); }
+  unsigned getLog2RISCVABIVLen() const {
+    return getExtInfo().getLog2RISCVABIVLen();
+  }
   ExtInfo getExtInfo() const { return ExtInfo(FunctionTypeBits.ExtInfo); }
 
   static_assert((~Qualifiers::FastMask & Qualifiers::CVRMask) == 0,

clang/include/clang/AST/TypeProperties.td

@@ -313,14 +313,17 @@ let Class = FunctionType in {
   def : Property<"cmseNSCall", Bool> {
     let Read = [{ node->getExtInfo().getCmseNSCall() }];
   }
+  def : Property<"Log2RISCVABIVLen", UInt32> {
+    let Read = [{ node->getExtInfo().getLog2RISCVABIVLen() }];
+  }
 }
 
 let Class = FunctionNoProtoType in {
   def : Creator<[{
     auto extInfo = FunctionType::ExtInfo(noReturn, hasRegParm, regParm,
                                          callingConvention, producesResult,
                                          noCallerSavedRegs, noCfCheck,
-                                         cmseNSCall);
+                                         cmseNSCall, Log2RISCVABIVLen);
     return ctx.getFunctionNoProtoType(returnType, extInfo);
   }]>;
 }
@@ -363,7 +366,7 @@ let Class = FunctionProtoType in {
     auto extInfo = FunctionType::ExtInfo(noReturn, hasRegParm, regParm,
                                          callingConvention, producesResult,
                                          noCallerSavedRegs, noCfCheck,
-                                         cmseNSCall);
+                                         cmseNSCall, Log2RISCVABIVLen);
     FunctionProtoType::ExtProtoInfo epi;
     epi.ExtInfo = extInfo;
     epi.Variadic = variadic;

clang/include/clang/Basic/Attr.td

@@ -3139,6 +3139,14 @@ def RISCVVectorCC: DeclOrTypeAttr, TargetSpecificAttr<TargetRISCV> {
  let Documentation = [RISCVVectorCCDocs];
 }
 
+def RISCVVLSCC: DeclOrTypeAttr, TargetSpecificAttr<TargetRISCV> {
+ let Spellings = [CXX11<"riscv", "vls_cc">,
+                  C23<"riscv", "vls_cc">,
+                  Clang<"riscv_vls_cc">];
+ let Args = [UnsignedArgument<"VectorWidth", /*opt*/1>];
+ let Documentation = [RISCVVLSCCDocs];
+}
+
 def Target : InheritableAttr {
   let Spellings = [GCC<"target">];
   let Args = [StringArgument<"featuresStr">];

clang/include/clang/Basic/AttrDocs.td

@@ -5554,6 +5554,17 @@ them if they use them.
  }];
 }
 
+def RISCVVLSCCDocs : Documentation {
+ let Category = DocCatCallingConvs;
+ let Heading = "riscv::vls_cc, riscv_vls_cc, clang::riscv_vls_cc";
+ let Content = [{
+The ``riscv_vls_cc`` attribute can be applied to a function. Functions
+declared with this attribute will utilize the standard fixed-length vector
+calling convention variant instead of the default calling convention defined by
+the ABI. This variant aims to pass fixed-length vectors via vector registers,
+if possible, rather than through general-purpose registers.}];
+}
+
 def PreferredNameDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{

clang/include/clang/Basic/Specifiers.h

@@ -297,6 +297,7 @@ namespace clang {
     CC_M68kRTD,           // __attribute__((m68k_rtd))
     CC_PreserveNone,      // __attribute__((preserve_none))
     CC_RISCVVectorCall,   // __attribute__((riscv_vector_cc))
+    CC_RISCVVLSCall,      // __attribute__((riscv_vls_cc))
   };
 
   /// Checks whether the given calling convention supports variadic

clang/include/clang/CodeGen/CGFunctionInfo.h

@@ -608,6 +608,9 @@ class CGFunctionInfo final
   /// Log 2 of the maximum vector width.
   unsigned MaxVectorWidth : 4;
 
+  /// Log2 of ABI_VLEN used in RISCV VLS calling convention.
+  unsigned Log2RISCVABIVLen : 4;
+
   RequiredArgs Required;
 
   /// The struct representing all arguments passed in memory.  Only used when
@@ -718,11 +721,13 @@ class CGFunctionInfo final
   bool getHasRegParm() const { return HasRegParm; }
   unsigned getRegParm() const { return RegParm; }
 
+  unsigned getLog2RISCVABIVLen() const { return Log2RISCVABIVLen; }
+
   FunctionType::ExtInfo getExtInfo() const {
     return FunctionType::ExtInfo(isNoReturn(), getHasRegParm(), getRegParm(),
                                  getASTCallingConvention(), isReturnsRetained(),
                                  isNoCallerSavedRegs(), isNoCfCheck(),
-                                 isCmseNSCall());
+                                 isCmseNSCall(), getLog2RISCVABIVLen());
   }
 
   CanQualType getReturnType() const { return getArgsBuffer()[0].type; }
@@ -776,6 +781,7 @@ class CGFunctionInfo final
     ID.AddInteger(RegParm);
     ID.AddBoolean(NoCfCheck);
     ID.AddBoolean(CmseNSCall);
+    ID.AddInteger(Log2RISCVABIVLen);
     ID.AddInteger(Required.getOpaqueData());
     ID.AddBoolean(HasExtParameterInfos);
     if (HasExtParameterInfos) {
@@ -803,6 +809,7 @@ class CGFunctionInfo final
     ID.AddInteger(info.getRegParm());
     ID.AddBoolean(info.getNoCfCheck());
     ID.AddBoolean(info.getCmseNSCall());
+    ID.AddInteger(info.getLog2RISCVABIVLen());
     ID.AddInteger(required.getOpaqueData());
     ID.AddBoolean(!paramInfos.empty());
     if (!paramInfos.empty()) {

clang/include/clang/Driver/Options.td

@@ -4865,6 +4865,8 @@ def mrvv_vector_bits_EQ : Joined<["-"], "mrvv-vector-bits=">, Group<m_Group>,
       !eq(GlobalDocumentation.Program, "Flang") : "",
       true: " The value will be reflected in __riscv_v_fixed_vlen preprocessor define"),
     " (RISC-V only)")>;
+def mriscv_abi_vlen_EQ : Joined<["-"], "mriscv-abi-vlen=">, Group<m_Group>,
+                         HelpText<"Specify the VLEN for VLS calling convention.">;
 
 def munaligned_access : Flag<["-"], "munaligned-access">, Group<m_Group>,
   HelpText<"Allow memory accesses to be unaligned (AArch32/MIPSr6 only)">;

clang/lib/AST/ASTContext.cpp

@@ -10826,6 +10826,8 @@ QualType ASTContext::mergeFunctionTypes(QualType lhs, QualType rhs,
     return {};
   if (lbaseInfo.getNoCfCheck() != rbaseInfo.getNoCfCheck())
     return {};
+  if (lbaseInfo.getLog2RISCVABIVLen() != rbaseInfo.getLog2RISCVABIVLen())
+    return {};
 
   // When merging declarations, it's common for supplemental information like
   // attributes to only be present in one of the declarations, and we generally

clang/lib/AST/ItaniumMangle.cpp

@@ -3452,6 +3452,7 @@ StringRef CXXNameMangler::getCallingConvQualifierName(CallingConv CC) {
   case CC_M68kRTD:
   case CC_PreserveNone:
   case CC_RISCVVectorCall:
+  case CC_RISCVVLSCall:
     // FIXME: we should be mangling all of the above.
     return "";