From 6d2c2094648e956352723113a55e8f3963c35ceb Mon Sep 17 00:00:00 2001
From: Guillaume Melquiond
Date: Sat, 26 Dec 2020 17:40:38 +0100
Subject: Protect caml_process_pending_actions_exn with caml_something_to_do.

When using OCaml >= 4.10, function caml_process_pending_actions_exn is
called whenever the bytecode interpreter tries to apply a term. This
function exits immediately when caml_something_to_do is not set. But since
term application happens every few opcodes, even exiting immediately still
accounts between 5% and 10% of the whole interpreter. So, this commit
makes sure the function is not called unless caml_something_to_do is
already set (i.e., when the user presses Ctrl+C). This means that this
conditional branch is perfectly predicted by the processor.

On the following benchmark, this commit makes the VM 13% faster.

  Time Eval vm_compute in Pos.iter (fun x => x) tt 100000000.

Note that, before OCaml 4.10, the VM code was checking the value of
caml_signals_are_pending before calling caml_process_pending_signals. So,
this commit actually fixes a regression.
---
 kernel/byterun/coq_interp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'kernel')

diff --git a/kernel/byterun/coq_interp.c b/kernel/byterun/coq_interp.c
index 6255250218..a825283b2b 100644
--- a/kernel/byterun/coq_interp.c
+++ b/kernel/byterun/coq_interp.c
@@ -551,7 +551,7 @@ value coq_interprete
       CHECK_STACK(0);
       /* We also check for signals */
 #if OCAML_VERSION >= 41000
-      {
+      if (caml_something_to_do) {
         value res = caml_process_pending_actions_exn();
         if (Is_exception_result(res)) {
           /* If there is an asynchronous exception, we reset the vm */
-- 
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