YJIT: Use u32 for CodePtr to save 4 bytes each

We've long had a size restriction on the code memory region such that a
u32 could refer to everything. This commit capitalizes on this
restriction by shrinking the size of `CodePtr` to be 4 bytes from 8.

To derive a full raw pointer from a `CodePtr`, one needs a base pointer.
Both `CodeBlock` and `VirtualMemory` can be used for this purpose. The
base pointer is readily available everywhere, except for in the case of
the `jit_return` "branch". Generalize lea_label() to lea_jump_target()
in the IR to delay deriving the `jit_return` address until `compile()`,
when the base pointer is available.

On railsbench, this yields roughly a 1% reduction to `yjit_alloc_size`
(58,397,765 to 57,742,248).

1 files changed, 15 insertions, 1 deletions

diff --git a/yjit/src/utils.rs b/yjit/src/utils.rs
index 58415e279f..6bc66ee33e 100644
--- a/yjit/src/utils.rs
+++ b/yjit/src/utils.rs
@@ -51,6 +51,20 @@ impl IntoUsize for u8 {
     }
 }
 
+/// The [Into<u64>] Rust does not provide.
+/// Convert to u64 with assurance that the value is preserved.
+/// Currently, `usize::BITS == 64` holds for all platforms we support.
+pub(crate) trait IntoU64 {
+    fn as_u64(self) -> u64;
+}
+
+#[cfg(target_pointer_width = "64")]
+impl IntoU64 for usize {
+    fn as_u64(self) -> u64 {
+        self as u64
+    }
+}
+
 /// Compute an offset in bytes of a given struct field
 #[allow(unused)]
 macro_rules! offset_of {
@@ -219,7 +233,7 @@ pub fn print_str(asm: &mut Assembler, str: &str) {
     asm.bake_string(str);
     asm.write_label(after_string);
 
-    let opnd = asm.lea_label(string_data);
+    let opnd = asm.lea_jump_target(string_data);
     asm.ccall(print_str_cfun as *const u8, vec![opnd, Opnd::UImm(str.len() as u64)]);
 
     asm.cpop_all();