1.0.9.30: Make SBCL work on MIPS/Linux with larger page sizes.

[sbcl.git] / OPTIMIZATIONS

diff --git a/OPTIMIZATIONS b/OPTIMIZATIONS
index 69f4447..45b5463 100644 (file)
--- a/OPTIMIZATIONS
+++ b/OPTIMIZATIONS
@@ -224,6 +224,12 @@ 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.
 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.

+
+(This is harder than it might look at first glance, as MAKE-ARRAY is smart
+enough to eliminate something like ':initial-element 0'.  Such an optimization
+is valid if the vector is being allocated in the heap, but not if it is being
+allocated on the stack.  You could remove this optimization, but that makes
+the heap-allocated case somewhat slower...)

 --------------------------------------------------------------------------------
 #28
 a. Accessing raw slots in structure instances is more inefficient than
 --------------------------------------------------------------------------------
 #28
 a. Accessing raw slots in structure instances is more inefficient than

@@ -347,4 +353,89 @@ arguments:
 
 Notice that '(MOD 46) has two entries in the constant vector.  Having
 one would be preferable.
 
 Notice that '(MOD 46) has two entries in the constant vector.  Having
 one would be preferable.

+--------------------------------------------------------------------------------
+#35
+Compiling
+
+(defun foo (a i)
+  (declare (type simple-vector a))
+  (aref a i))
+
+results in the following x86 code:
+
+; 115886E9:       F7C703000000     TEST EDI, 3                ; no-arg-parsing entry point
+;      6EF:       7510             JNE L0
+;      6F1:       8BC7             MOV EAX, EDI
+;      6F3:       83F800           CMP EAX, 0
+;      6F6:       7C09             JL L0
+;      6F8:       8BC7             MOV EAX, EDI
+;      6FA:       3DF8FFFF7F       CMP EAX, 2147483640
+;      6FF:       7E0F             JLE L1
+;      701: L0:   8B057C865811     MOV EAX, [#x1158867C]      ; '(MOD
+                                                              ;   536870911)
+;      707:       0F0B0A           BREAK 10                   ; error trap
+;      70A:       05               BYTE #X05
+;      70B:       1F               BYTE #X1F                  ; OBJECT-NOT-TYPE-ERROR
+;      70C:       FECE01           BYTE #XFE, #XCE, #X01      ; EDI
+;      70F:       0E               BYTE #X0E                  ; EAX
+;      710: L1:   8B42FD           MOV EAX, [EDX-3]
+;      713:       8BCF             MOV ECX, EDI
+;      715:       39C8             CMP EAX, ECX
+;      717:       7620             JBE L2
+;      719:       8B540A01         MOV EDX, [EDX+ECX+1]
+
+... plus the standard return sequence and some error blocks.  The
+`TEST EDI, 3' and associated comparisons are to ensure that `I' is a
+positive fixnum.  The associated comparisons are unnecessary, as the
+%CHECK-BOUND VOP only requires its tested index to be a fixnum and takes
+care of the negative fixnum case itself.
+
+{HAIRY-,}DATA-VECTOR-REF are DEFKNOWN'd with EXPLICIT-CHECK, which would
+seem to take care of this, but EXPLICIT-CHECK only seems to be used when
+compiling calls to unknown functions or similar.  Furthermore,
+EXPLICIT-CHECK, as NJF understands it, doesn't have the right
+semantics--it suppresses all type checking of arguments, whereas what we
+really want is to ensure that the argument is a fixnum, but not check
+its positiveness.
+--------------------------------------------------------------------------------
+#36
+
+In #35, the CMP EAX, $foo instructions are all preceded by a MOV.  They
+appear to be unnecessary, but are necessary because in IR2, EDI is a
+DESCRIPTOR-REG, whereas EAX is an ANY-REG--and the comparison VOPs only
+accept ANY-REGs.  Therefore, the MOVs are "necessary" to ensure that the
+comparison VOP receives an TN of the appropriate storage class.
+
+Obviously, it would be better if a) we only performed one MOV prior to
+all three comparisons or b) eliminated the necessity of the MOV(s)
+altogether.  The former option is probably easier than the latter.
+
+--------------------------------------------------------------------------------
+#37
+
+Dynamic extent allocation doesn't currently work for one-element lists,
+since there's a source transform from (LIST X) to (CONS X NIL).
+
+--------------------------------------------------------------------------------
+#38
+
+(setf (subseq s1 start1 end1) (subseq s2 start2 end1))
+
+could be transformed into
+
+(let ((#:s2 s2)
+      (#:start2 start2)
+      (#:end2 end2))
+ (replace s1 #:s2 :start1 start1 :end1 end1 :start2 #:start2 :end2 #:end2))
+
+when the return value is unused, avoiding the need to cons up the new sequence.
+
+--------------------------------------------------------------------------------
+#39
+
+(let ((*foo* 42)) ...)
+
+currently compiles to code that ensures the TLS index at runtime, which
+is both a decently large chunk of code and unnecessary, as we could ensure
+the TLS index at load-time as well.