From: Taylor R Campbell <campbell@mumble.net>
Date: Sun, 26 Jun 2011 19:39:09 +0000 (+0000)
Subject: Move interrupt branch from start to end of block, on i386.
X-Git-Tag: release-9.1.0~3^2~4
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=3ecdecd3fe9e8050f75e7eecab1011896ea9c601;p=mit-scheme.git

Move interrupt branch from start to end of block, on i386.

This makes it a forward branch rather than a reverse branch, so that
the CPU's branch predictor will predict it not taken rather than taken
by default.

Reduces time to compile the whole system by about 20%.  This
performance increase will apply to any compiled code, independent of
what sort of computation it does.
---

diff --git a/src/compiler/machines/i386/rules3.scm b/src/compiler/machines/i386/rules3.scm
index a6da4e2da..c9da18f50 100644
--- a/src/compiler/machines/i386/rules3.scm
+++ b/src/compiler/machines/i386/rules3.scm
@@ -398,14 +398,23 @@ USA.
 
 (define (interrupt-check interrupt-label checks)
   ;; This always does interrupt checks in line.
-  (LAP ,@(if (or (memq 'INTERRUPT checks) (memq 'HEAP checks))
-	     (LAP (CMP W (R ,regnum:free-pointer) ,reg:compiled-memtop)
-		  (JGE (@PCR ,interrupt-label)))
-	     (LAP))
-       ,@(if (memq 'STACK checks)
-	     (LAP (CMP W (R ,regnum:stack-pointer) ,reg:stack-guard)
-		  (JL (@PCR ,interrupt-label)))
-	     (LAP))))
+  (let ((branch-target (generate-label 'INTERRUPT)))
+    ;; Put the interrupt check branch target after the branch so that
+    ;; it is a forward branch, which Intel and AMD CPUs will predict
+    ;; not taken by default, in the absence of dynamic branch
+    ;; prediction profile data.
+    (add-end-of-block-code!
+     (lambda ()
+       (LAP (LABEL ,branch-target)
+	    (JMP (@PCR ,interrupt-label)))))
+    (LAP ,@(if (or (memq 'INTERRUPT checks) (memq 'HEAP checks))
+	       (LAP (CMP W (R ,regnum:free-pointer) ,reg:compiled-memtop)
+		    (JGE (@PCR ,branch-target)))
+	       (LAP))
+	 ,@(if (memq 'STACK checks)
+	       (LAP (CMP W (R ,regnum:stack-pointer) ,reg:stack-guard)
+		    (JL (@PCR ,branch-target)))
+	       (LAP)))))
 
 (define (simple-procedure-header code-word label entry)
   (let ((checks (get-entry-interrupt-checks)))