LLVM: implement signext/zeroext attributes

For calling convention ABI purposes, integer attributes and return
values need to have an LLVM attribute signext or zeroext added
sometimes. This commit implements that logic.

It also implements a proof-of-concept of moving the F16T type from
being a compiler_rt hack to being how the compiler lowers f16 in
functions that need to match certain calling conventions.

Closes #12054

src/codegen/llvm.zig

@@ -717,6 +717,11 @@ pub const Object = struct {
         const ret_ptr = if (sret) llvm_func.getParam(0) else null;
         const gpa = dg.gpa;
 
+        if (ccAbiPromoteInt(fn_info.cc, target, fn_info.return_type)) |s| switch (s) {
+            .signed => dg.addAttr(llvm_func, 0, "signext"),
+            .unsigned => dg.addAttr(llvm_func, 0, "zeroext"),
+        };
+
         const err_return_tracing = fn_info.return_type.isError() and
             dg.module.comp.bin_file.options.error_return_tracing;
 
@@ -774,7 +779,10 @@ pub const Object = struct {
                                 );
                                 dg.addArgAttrInt(llvm_func, llvm_arg_i, "align", elem_align);
                             }
-                        }
+                        } else if (ccAbiPromoteInt(fn_info.cc, target, param_ty)) |s| switch (s) {
+                            .signed => dg.addArgAttr(llvm_func, llvm_arg_i, "signext"),
+                            .unsigned => dg.addArgAttr(llvm_func, llvm_arg_i, "zeroext"),
+                        };
                     }
                     llvm_arg_i += 1;
                 },
@@ -887,6 +895,13 @@ pub const Object = struct {
                     };
                     try args.append(loaded);
                 },
+                .as_u16 => {
+                    const param = llvm_func.getParam(llvm_arg_i);
+                    llvm_arg_i += 1;
+                    const casted = builder.buildBitCast(param, dg.context.halfType(), "");
+                    try args.ensureUnusedCapacity(1);
+                    args.appendAssumeCapacity(casted);
+                },
             };
         }
 
@@ -2794,6 +2809,9 @@ pub const DeclGen = struct {
                     llvm_params.appendAssumeCapacity(big_int_ty);
                 }
             },
+            .as_u16 => {
+                try llvm_params.append(dg.context.intType(16));
+            },
         };
 
         return llvm.functionType(
@@ -4234,6 +4252,12 @@ pub const FuncGen = struct {
                     llvm_args.appendAssumeCapacity(load_inst);
                 }
             },
+            .as_u16 => {
+                const arg = args[it.zig_index - 1];
+                const llvm_arg = try self.resolveInst(arg);
+                const casted = self.builder.buildBitCast(llvm_arg, self.dg.context.intType(16), "");
+                try llvm_args.append(casted);
+            },
         };
 
         const call = self.builder.buildCall(
@@ -8965,6 +8989,7 @@ const ParamTypeIterator = struct {
         abi_sized_int,
         multiple_llvm_ints,
         slice,
+        as_u16,
     };
 
     pub fn next(it: *ParamTypeIterator) ?Lowering {
@@ -9025,6 +9050,15 @@ const ParamTypeIterator = struct {
                     else => false,
                 };
                 switch (it.target.cpu.arch) {
+                    .riscv32, .riscv64 => {
+                        it.zig_index += 1;
+                        it.llvm_index += 1;
+                        if (ty.tag() == .f16) {
+                            return .as_u16;
+                        } else {
+                            return .byval;
+                        }
+                    },
                     .mips, .mipsel => {
                         it.zig_index += 1;
                         it.llvm_index += 1;
@@ -9135,6 +9169,35 @@ fn iterateParamTypes(dg: *DeclGen, fn_info: Type.Payload.Function.Data) ParamTyp
     };
 }
 
+fn ccAbiPromoteInt(
+    cc: std.builtin.CallingConvention,
+    target: std.Target,
+    ty: Type,
+) ?std.builtin.Signedness {
+    switch (cc) {
+        .Unspecified, .Inline, .Async => return null,
+        else => {},
+    }
+    const int_info = switch (ty.zigTypeTag()) {
+        .Int, .Enum, .ErrorSet => ty.intInfo(target),
+        else => return null,
+    };
+    if (int_info.bits <= 16) return int_info.signedness;
+    switch (target.cpu.arch) {
+        .sparc64,
+        .riscv64,
+        .powerpc64,
+        .powerpc64le,
+        => {
+            if (int_info.bits < 64) {
+                return int_info.signedness;
+            }
+        },
+        else => {},
+    }
+    return null;
+}
+
 fn isByRef(ty: Type) bool {
     // For tuples and structs, if there are more than this many non-void
     // fields, then we make it byref, otherwise byval.