From 88549ec38e9bb30e338a9985d0de4e6263b40fb7 Mon Sep 17 00:00:00 2001
From: Vibhav Pant
Date: Sun, 15 Jan 2017 01:26:04 +0530
Subject: Add new 'switch' byte-code.

'switch' takes two arguments from the stack: the variable to test, and
a jump table (implemented as a hash-table with the appropriate :test
function). By looking up the value of the variable in the hash table,
the interpreter can jump to the label pointed to by the value, if any.
This implementation can only be used for `cond' forms of the type
`(cond ((test x 'foo) 'bar) ...)`, such that the function `test` and
variable `x` is same for all clauses.

* lisp/emacs-lisp/bytecomp.el:

  * Add (byte-compile-cond-valid-obj2-p), (byte-compile-cond-vars),
    (byte-compile-cond-jump-table-info), (byte-compile-jump-table-add-tag),
    (byte-compile-cond-jump-table), byte-compile-jump-tables.

  * Add defcustom `byte-compile-cond-use-jump-table'.

  * (byte-compile-cond): Use them.

  * (byte-compile-lapcode): Patch tags present in jump tables, if any.

* lisp/emacs-lisp//byte-opt.el: (byte-optimize-lapcode): Add checks to
  some peephole optimizations to prevent them from messing up any code
  involving `byte-switch`.

* src/bytecode.c: (exec_byte_code): Add bytecode Bswitch.
---
 src/bytecode.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

(limited to 'src/bytecode.c')

diff --git a/src/bytecode.c b/src/bytecode.c
index a64bc171d14..1695af9cb02 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -267,6 +267,8 @@ DEFINE (Bstack_set,  0262)						\
 DEFINE (Bstack_set2, 0263)						\
 DEFINE (BdiscardN,   0266)						\
 									\
+DEFINE (Bswitch, 0267)                                                  \
+                                                                        \
 DEFINE (Bconstant, 0300)
 
 enum byte_code_op
@@ -1411,6 +1413,20 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
 	  DISCARD (op);
 	  NEXT;
 
+        CASE (Bswitch):
+          {
+            Lisp_Object jmp_table = POP;
+            Lisp_Object v1 = POP;
+            Lisp_Object dest = Fgethash(v1, jmp_table, Qnil);
+            if (!NILP(dest)) {
+              int car = XINT(XCAR(dest));
+              int cdr = XINT(XCDR(dest));
+              op = car + (cdr << 8); /* Simulate FETCH2 */
+              goto op_branch;
+            }
+          }
+          NEXT;
+
 	CASE_DEFAULT
 	CASE (Bconstant):
 	  if (BYTE_CODE_SAFE
-- 
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