Add a spiderwasm_prologue_words setting.

This makes the details of the spiderwasm prologue configurable so it is
easier to modify SpiderMonkey without having to change Cretonne.

Create a stack object representing the SpiderMonkey prologue words
before calculating the stack layout so they won't be overwritten by
Cretonne's stack objects.

lib/cretonne/meta/base/settings.py

@@ -4,7 +4,7 @@ Cretonne shared settings.
 This module defines settings relevant for all code generators.
 """
 from __future__ import absolute_import
-from cdsl.settings import SettingGroup, BoolSetting, EnumSetting
+from cdsl.settings import SettingGroup, BoolSetting, EnumSetting, NumSetting
 
 group = SettingGroup('shared')
 
@@ -52,4 +52,22 @@ enable_atomics = BoolSetting(
         """Enable the use of atomic instructions""",
         default=True)
 
+#
+# Settings specific to the `spiderwasm` calling convention.
+#
+spiderwasm_prologue_words = NumSetting(
+        """
+        Number of pointer-sized words pushed by the spiderwasm prologue.
+
+        Functions with the `spiderwasm` 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.
+
+        This setting configures the number of pointer-sized words pushed on the
+        stack when the Cretonne-generated code is entered. This includes the
+        pushed return address on Intel ISAs.
+        """)
+
+
 group.close(globals())

lib/cretonne/src/isa/mod.rs

@@ -236,9 +236,19 @@ pub trait TargetIsa {
     fn prologue_epilogue(&self, func: &mut ir::Function) -> result::CtonResult {
         // This default implementation is unlikely to be good enough.
         use stack_layout::layout_stack;
+        use ir::stackslot::{StackSize, StackOffset};
 
-        let align = if self.flags().is_64bit() { 8 } else { 4 };
-        layout_stack(&mut func.stack_slots, align)?;
+        let word_size = if self.flags().is_64bit() { 8 } else { 4 };
+
+        // Account for the SpiderMonkey standard prologue pushes.
+        if func.signature.call_conv == ir::CallConv::SpiderWASM {
+            let bytes = self.flags().spiderwasm_prologue_words() as StackSize * word_size;
+            let mut ss = ir::StackSlotData::new(ir::StackSlotKind::IncomingArg, bytes);
+            ss.offset = -(bytes as StackOffset);
+            func.stack_slots.push(ss);
+        }
+
+        layout_stack(&mut func.stack_slots, word_size)?;
         Ok(())
     }
 

lib/cretonne/src/settings.rs

@@ -323,10 +323,12 @@ mod tests {
                     is_compressed = false\n\
                     enable_float = true\n\
                     enable_simd = true\n\
-                    enable_atomics = true\n"
+                    enable_atomics = true\n\
+                    spiderwasm_prologue_words = 0\n"
         );
         assert_eq!(f.opt_level(), super::OptLevel::Default);
         assert_eq!(f.enable_simd(), true);
+        assert_eq!(f.spiderwasm_prologue_words(), 0);
     }
 
     #[test]