Refactor calling convention settings. (#304)

Add a calling-convention setting to the `Flags` used as part of the
`TargetIsa`. This allows Cretonne code that generates calls to use the
correct convention, such as when emitting libcalls during legalization
or when the wasm frontend is decoding functions. This setting can be
overridden per-function.

This also adds "fast", "cold", and "fastcall" conventions, with "fast"
as the new default. Note that "fast" and "cold" are not intended to be
ABI-compatible across Cretonne versions.

This will also ensure Windows users will get an `unimplemented!` rather
than silent calling-convention mismatches, which reflects the fact that
Windows calling conventions are not yet implemented.

This also renames SpiderWASM, which isn't camel-case, to Baldrdash,
which is, and which is also a more relevant name.

lib/codegen/meta/base/settings.py

@@ -29,6 +29,21 @@ enable_verifier = BoolSetting(
 
 is_64bit = BoolSetting("Enable 64-bit code generation")
 
+call_conv = EnumSetting(
+        """
+        Default calling convention:
+
+        - fast: not-ABI-stable convention for best performance
+        - cold: not-ABI-stable convention for infrequently executed code
+        - system_v: System V-style convention used on many platforms
+        - fastcall: Windows "fastcall" convention, also used for x64 and ARM
+        - baldrdash: SpiderMonkey WebAssembly convention
+
+        The default calling convention may be overridden by individual
+        functions.
+        """,
+        'fast', 'cold', 'system_v', 'fastcall', 'baldrdash')
+
 # Note that Cretonne doesn't currently need an is_pie flag, because PIE is just
 # PIC where symbols can't be pre-empted, which can be expressed with the
 # `colocated` flag on external functions and global variables.
@@ -75,13 +90,13 @@ enable_atomics = BoolSetting(
         default=True)
 
 #
-# Settings specific to the `spiderwasm` calling convention.
+# Settings specific to the `baldrdash` calling convention.
 #
-spiderwasm_prologue_words = NumSetting(
+baldrdash_prologue_words = NumSetting(
         """
-        Number of pointer-sized words pushed by the spiderwasm prologue.
+        Number of pointer-sized words pushed by the baldrdash prologue.
 
-        Functions with the `spiderwasm` calling convention don't generate their
+        Functions with the `baldrdash` calling convention don't generate their
         own prologue and epilogue. They depend on externally generated code
         that pushes a fixed number of words in the prologue and restores them
         in the epilogue.