In aliencomp.c #+(and ppc darwin) should be #!+(and ppc darwin), which

[sbcl.git] / OPTIMIZATIONS

diff --git a/OPTIMIZATIONS b/OPTIMIZATIONS
index eb2931f..3795bf4 100644 (file)
--- a/OPTIMIZATIONS
+++ b/OPTIMIZATIONS
@@ -224,3 +224,51 @@ Initialization of stack-allocated arrays is inefficient: we always
 fill the vector with zeroes, even when it is not needed (as for
 platforms with conservative GC or for arrays of unboxed objectes) and
 is performed later explicitely.
+--------------------------------------------------------------------------------
+#28
+a. Accessing raw slots in structure instances is more inefficient than
+it could be; if we placed raw slots before the header word, we would
+not need to do arithmetic at runtime to access them.  (But beware:
+this would complicate handling of the interior pointer).
+
+b. (Also note that raw slots are currently disabled on HPPA)
+--------------------------------------------------------------------------------
+#29
+Python is overly zealous when converting high-level CL functions, such
+as MIN/MAX, LOGBITP, and LOGTEST, to low-level CL functions.  Reducing
+Python's aggressiveness would make it easier to effect changes such as
+
+x86-64:
+* direct MIN/MAX on {SINGLE,DOUBLE}-FLOATs ({MIN,MAX}S{S,D})
+
+x86-64:
+* direct LOGBITP on word-sized integers and fixnums (BT + JC)
+
+x86{,-64}/PPC:
+* branch-free MIN/MAX on word-sized integers and fixnums (floats could
+  be handled too, modulo safety considerations on the PPC)
+
+x86-64:
+* efficient LOGTESTs on word-sized integers and fixnums (TEST)
+
+etc., etc.
+
+(The framework for this has been implemented as of 0.9.9.18; see the
+vm-support-routine COMBINATION-IMPLEMENTATION-STYLE and its use in
+src/compiler/ir1opt.lisp, IR1-OPTIMIZE-COMBINATION.  The above
+optimizations are left as an exercise for the reader.)
+--------------------------------------------------------------------------------
+#30
+(defun foo (x y)
+  (< x y))
+
+FOO's IR1 representation is roughly:
+
+(defun foo (x y)
+  (if (< x y)
+      T
+      NIL))
+
+However, if a full call is generated for < (and similarly for other
+predicate functions), then the IF is unnecessary, since the return value
+of (< x y) is already T or NIL.