Windows FPRs preservation (#1216)

Preserve FPRs as required by the Windows fastcall calling convention.

This exposes an implementation limit due to Cranelift's approach to stack layout, which conflicts with expectations Windows makes in SEH layout - functions where the Cranelift user desires fastcall unwind information, that require preservation of an ABI-reserved FPR, that have a stack frame 240 bytes or larger, now produce an error when compiled. Several wasm spectests were disabled because they would trip this limit. This is a temporary constraint that should be fixed promptly.

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

crates/environ/src/compilation.rs

@@ -4,7 +4,7 @@
 use crate::cache::ModuleCacheDataTupleType;
 use crate::CacheConfig;
 use crate::ModuleTranslation;
-use cranelift_codegen::{binemit, ir, isa, Context};
+use cranelift_codegen::{binemit, ir, isa, CodegenResult, Context};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{DefinedFuncIndex, FuncIndex, WasmError};
 use serde::{Deserialize, Serialize};
@@ -36,7 +36,7 @@ pub enum CompiledFunctionUnwindInfo {
 
 impl CompiledFunctionUnwindInfo {
     /// Constructs unwind info object.
-    pub fn new(isa: &dyn isa::TargetIsa, context: &Context) -> Self {
+    pub fn new(isa: &dyn isa::TargetIsa, context: &Context) -> CodegenResult<Self> {
         use cranelift_codegen::binemit::{
             FrameUnwindKind, FrameUnwindOffset, FrameUnwindSink, Reloc,
         };
@@ -75,24 +75,26 @@ impl CompiledFunctionUnwindInfo {
             CallConv::SystemV | CallConv::Fast | CallConv::Cold => FrameUnwindKind::Libunwind,
             CallConv::WindowsFastcall => FrameUnwindKind::Fastcall,
             _ => {
-                return CompiledFunctionUnwindInfo::None;
+                return Ok(CompiledFunctionUnwindInfo::None);
             }
         };
 
         let mut sink = Sink(Vec::new(), 0, Vec::new());
-        context.emit_unwind_info(isa, kind, &mut sink);
+        context.emit_unwind_info(isa, kind, &mut sink)?;
 
         let Sink(data, offset, relocs) = sink;
         if data.is_empty() {
-            return CompiledFunctionUnwindInfo::None;
+            return Ok(CompiledFunctionUnwindInfo::None);
         }
 
-        match kind {
+        let info = match kind {
             FrameUnwindKind::Fastcall => CompiledFunctionUnwindInfo::Windows(data),
             FrameUnwindKind::Libunwind => {
                 CompiledFunctionUnwindInfo::FrameLayout(data, offset, relocs)
             }
-        }
+        };
+
+        Ok(info)
     }
 
     /// Retuns true is no unwind info data.