#20: Modular arith bug? --------------------+------------------------------------------------------- Reporter: rtoy | Owner: somebody Type: defect | Status: new Priority: major | Milestone: Component: Core | Version: 19e Keywords: | --------------------+------------------------------------------------------- {{{ (defun mat3neg (tt v) (ldb (byte 32 0) (ash v (- tt))))
(defun mat3neg-a (tt v) (logand #xffffffff (ash v (- tt))))
(defun zot (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg -28 z2))
(defun zot-a (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg-a -28 z2)) }}}
Compile {{{zot}}} and there are lots of compiler notes, from {{{mat3neg}}}. But compile {{{zot-a}}} and there's just one for boxing up the result.
However, {{{zot}}} and {{{zot-a}}} are functionally identical, so {{{zot}}} and {{{zot-a}}} should produce the same code. But {{{zot}}} has to do a full call to {{{ash}}} and {{{two-arg-and}}}.
The {{{ldb}}} must be confusing the compiler somehow.
#20: Modular arith bug? ---------------------+------------------------------------------------------ Reporter: rtoy | Owner: somebody Type: defect | Status: new Priority: major | Milestone: Component: Core | Version: 19e Resolution: | Keywords: ---------------------+------------------------------------------------------ Old description:
{{{ (defun mat3neg (tt v) (ldb (byte 32 0) (ash v (- tt))))
(defun mat3neg-a (tt v) (logand #xffffffff (ash v (- tt))))
(defun zot (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg -28 z2))
(defun zot-a (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg-a -28 z2)) }}}
Compile {{{zot}}} and there are lots of compiler notes, from {{{mat3neg}}}. But compile {{{zot-a}}} and there's just one for boxing up the result.
However, {{{zot}}} and {{{zot-a}}} are functionally identical, so {{{zot}}} and {{{zot-a}}} should produce the same code. But {{{zot}}} has to do a full call to {{{ash}}} and {{{two-arg-and}}}.
The {{{ldb}}} must be confusing the compiler somehow.
New description:
{{{ (declaim (inline mat3neg mat3neg-a)) (defun mat3neg (tt v) (ldb (byte 32 0) (ash v (- tt))))
(defun mat3neg-a (tt v) (logand #xffffffff (ash v (- tt))))
(defun zot (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg -28 z2))
(defun zot-a (z2) (declare (type (unsigned-byte 32) z2) (optimize speed (safety 0))) (mat3neg-a -28 z2)) }}}
Compile {{{zot}}} and there are lots of compiler notes, from {{{mat3neg}}}. But compile {{{zot-a}}} and there's just one for boxing up the result.
However, {{{zot}}} and {{{zot-a}}} are functionally identical, so {{{zot}}} and {{{zot-a}}} should produce the same code. But {{{zot}}} has to do a full call to {{{ash}}} and {{{two-arg-and}}}.
The {{{ldb}}} must be confusing the compiler somehow.
#20: Modular arith bug? ---------------------+------------------------------------------------------ Reporter: rtoy | Owner: somebody Type: defect | Status: new Priority: major | Milestone: Component: Core | Version: 19e Resolution: | Keywords: ---------------------+------------------------------------------------------ Comment (by rtoy):
The issue is partly caused by ldb and, perhaps, partly by ash modular function optimizer.
(ldb (byte size posn) x) is source-transformed to (%ldb size posn x), which has a deftransform that converts it to (logand (ash x (- posn)) (ash (1- (ash 1 vm:word-bits)) (- size vm:word-bits)).
This expression must confuse logand-defopt-helper. One workaround is to change the %ldb to produce simpler expressions when posn and size are known constants.
#20: Modular arith bug? ---------------------+------------------------------------------------------ Reporter: rtoy | Owner: somebody Type: defect | Status: closed Priority: major | Milestone: Component: Core | Version: 19e Resolution: fixed | Keywords: ---------------------+------------------------------------------------------ Changes (by rtoy):
* resolution: => fixed * status: new => closed
Comment:
This particular issue is fixed and should work as expected in the 2008-09 snapshot. The workaround was to change %ldb to optimize the expression at compile-time as much as possible.
Closing this particular issue.
-
cmucl