[pypy-svn] r37080 - pypy/dist/pypy/jit/codegen/i386

[arigo at codespeak.net]

Author: arigo
Date: Sat Jan 20 23:36:37 2007
New Revision: 37080

Modified:
   pypy/dist/pypy/jit/codegen/i386/rgenop.py
Log:
(pedronis, arigo)

Avoid keeping the JMP to the next block if it happens to be just after
the current block.  Big big optimization in practice.


Modified: pypy/dist/pypy/jit/codegen/i386/rgenop.py
==============================================================================
--- pypy/dist/pypy/jit/codegen/i386/rgenop.py	(original)
+++ pypy/dist/pypy/jit/codegen/i386/rgenop.py	Sat Jan 20 23:36:37 2007
@@ -1267,6 +1267,7 @@
         self.coming_from = mc.tell()
         insnemit = EMIT_JCOND[insncond]
         insnemit(mc, rel32(0))
+        self.coming_from_end = mc.tell()
 
     def start_mc(self):
         mc = self.rgenop.open_mc()
@@ -1274,13 +1275,23 @@
         start = self.coming_from
         if start:
             targetaddr = mc.tell()
+            end = self.coming_from_end
+            fallthrough = targetaddr == end
             if self.update_defaultcaseaddr_of:   # hack for FlexSwitch
                 self.update_defaultcaseaddr_of.defaultcaseaddr = targetaddr
-            end = start + 6    # XXX hard-coded, enough for JMP and Jcond
-            oldmc = self.rgenop.InMemoryCodeBuilder(start, end)
-            insn = EMIT_JCOND[self.coming_from_cond]
-            insn(oldmc, rel32(targetaddr))
-            oldmc.done()
+                fallthrough = False
+            if fallthrough:
+                # the jump would be with an offset 0, i.e. it would go
+                # exactly after itself, so we don't really need the jump
+                # instruction at all and we can overwrite it and continue.
+                mc.seekback(end - start)
+                targetaddr = start
+            else:
+                # normal case: patch the old jump to go to targetaddr
+                oldmc = self.rgenop.InMemoryCodeBuilder(start, end)
+                insn = EMIT_JCOND[self.coming_from_cond]
+                insn(oldmc, rel32(targetaddr))
+                oldmc.done()
             self.coming_from = 0
         return mc