Handle special-purpose arguments in the Intel ABI.

The VMContext and SignatureId arguments are passed in fixed registers
for the spiderwasm calling convention.

lib/cretonne/src/isa/intel/abi.rs

@@ -6,7 +6,7 @@ use regalloc::AllocatableSet;
 use settings as shared_settings;
 use super::registers::{GPR, FPR, RU};
 use abi::{ArgAction, ValueConversion, ArgAssigner, legalize_args};
-use ir::{ArgumentType, ArgumentLoc, ArgumentExtension};
+use ir::{ArgumentType, ArgumentPurpose, ArgumentLoc, ArgumentExtension};
 
 /// Argument registers for x86-64
 static ARG_GPRS: [RU; 6] = [RU::rdi, RU::rsi, RU::rdx, RU::rcx, RU::r8, RU::r9];
@@ -64,6 +64,24 @@ impl ArgAssigner for Args {
             }
         }
 
+        // Handle special-purpose arguments.
+        // TODO: The registers below are for `spiderwasm`. Should we check the calling convention?
+        if ty.is_int() {
+            match arg.purpose {
+                // This is SpiderMonkey's `WasmTlsReg`.
+                ArgumentPurpose::VMContext => {
+                    return ArgumentLoc::Reg(if self.pointer_bits == 64 {
+                        RU::r14
+                    } else {
+                        RU::rsi
+                    } as RegUnit).into()
+                }
+                // This is SpiderMonkey's `WasmTableCallSigReg`.
+                ArgumentPurpose::SignatureId => return ArgumentLoc::Reg(RU::rbx as RegUnit).into(),
+                _ => {}
+            }
+        }
+
         // Try to use a GPR.
         if !ty.is_float() && self.gpr_used < self.gpr.len() {
             let reg = self.gpr[self.gpr_used] as RegUnit;