YJIT: On-demand executable memory allocation; faster boot (#5944)

This commit makes YJIT allocate memory for generated code gradually as
needed. Previously, YJIT allocates all the memory it needs on boot in
one go, leading to higher than necessary resident set size (RSS) and
time spent on boot initializing the memory with a large memset().

Users should no longer need to search for a magic number to pass to
`--yjit-exec-mem` since physical memory consumption should now more
accurately reflect the requirement of the workload.

YJIT now reserves a range of addresses on boot. This region start out
with no access permission at all so buggy attempts to jump to the region
crashes like before this change. To get this hardening at finer
granularity than the page size, we fill each page with trapping
instructions when we first allocate physical memory for the page.

Most of the time applications don't need 256 MiB of executable code, so
allocating on-demand ends up doing less total work than before. Case in
point, a simple `ruby --yjit-call-threshold=1 -eitself` takes about
half as long after this change. In terms of memory consumption, here is
a table to give a rough summary of the impact:

    | Peak RSS in MiB | -eitself example | railsbench once |
    | :-------------: | ---------------: | --------------: |
    |     before      |              265 |             377 |
    |      after      |               11 |             143 |
    |     no YJIT     |               10 |             101 |

A new module is introduced to handle allocation bookkeeping.
`CodePtr` is moved into the module since it has a close relationship
with the new `VirtualMemory` struct. This new interface has a slightly
smaller surface than before in that marking a region as writable is no
longer a public operation.

1 files changed, 5 insertions, 1 deletions

diff --git a/yjit/src/stats.rs b/yjit/src/stats.rs
index e129cc2811..6bad8db7e7 100644
--- a/yjit/src/stats.rs
+++ b/yjit/src/stats.rs
@@ -141,7 +141,7 @@ macro_rules! incr_counter {
     ($counter_name:ident) => {
         #[allow(unused_unsafe)]
         {
-            unsafe { COUNTERS.$counter_name += 1 }
+            unsafe { $crate::stats::COUNTERS.$counter_name += 1 }
         }
     };
 }
@@ -244,6 +244,10 @@ make_counters! {
 
     gbpp_block_param_modified,
     gbpp_block_handler_not_iseq,
+
+    // Currently, it's out of the ordinary (might be impossible) for YJIT to leave gaps in
+    // executable memory, so this should be 0.
+    exec_mem_non_bump_alloc,
 }
 
 //===========================================================================