Replace uses of a variable aliasing another variable with that aliased
variable, to allow for variable removal when possible. This also
enables opportunities for other optimisations. Example:
(let ((y x)) (f y)) => (f x)
The optimisation is only performed if both aliased and aliasing
variables are lexically bound. Shadowing bindings are α-renamed when
necessary for correctness. Example:
(let* ((b a) (a EXPR)) (f a b))
=> (let* ((a{new} EXPR)) (f a{new} a))
* lisp/emacs-lisp/byte-opt.el (byte-optimize--aliased-vars): New.
(byte-optimize-form-code-walker): Cancel aliasing upon mutation.
(byte-optimize--rename-var-body, byte-optimize--rename-var): New.
(byte-optimize-let-form): Add the optimisation.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add relevant test cases.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-body): Flatten body.
This simplifies the source tree and reduces the number of different
cases that other optimisations need to take into account.
Treat `ignore` as any other function during source-level optimisation,
to avoid having its warning-suppression effects cancelled by repeated
passes. Instead, define a custom code generation function.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Don't treat `ignore' specially here.
(side-effect-free-fns): Don't mark `ignore` as side-effect-free
or error-free (although it is), since that would allow the optimiser
to elide calls.
* lisp/emacs-lisp/bytecomp.el (ignore, byte-compile-ignore):
Define and register a code-gen function.
* lisp/emacs-lisp/byte-opt.el
(byte-optimize-assq): New.
(byte-optimize-member, byte-optimize-assoc, byte-optimize-memq):
When the list argument is constant nil, the result is always nil.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.
The current method of propagating constants through setq was unsound
because it relied on each setq form only being traversed at most once
during optimisation, which isn't necessarily true in general; it could
be made to miscompile code in rare cases.
Since it was only used in limited circumstances, disabling this
optimisation doesn't cost us much.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Don't update the known value when traversing `setq`.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test case.
Ensure in cconv that let-bindings have the normal form (VAR EXPR)
where VAR is a valid variable name, so that we don't need to keep
re-checking this all the time in the optimiser.
* lisp/emacs-lisp/byte-opt.el
(byte-optimize-enable-variable-constprop)
(byte-optimize-warn-eliminated-variable): Remove; these were mainly
used for debugging.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form):
Assume normalised let-bindings (with lexical-binding).
Stop using the variables removed above.
* lisp/emacs-lisp/cconv.el (cconv-convert): Ensure normalised
let-bindings. Malformed bindings are dropped after warning.
remove byte-optimize-warn-eliminated-variable
Simplify (let ((X1 E1) ... (Xn En)) Xn)
=> (progn E1 ... En)
and (let* ((X1 E1) ... (Xn En)) Xn)
=> (let* ((X1 E1) ... (Xn-1 En-1)) En)
and similarly the case where the body is a constant, extending a
previous optimisation that only applied to the constant nil.
This reduces the number of bound variables, shortens the code, and
enables further optimisations.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-letX): Rewrite using
`pcase` and add the aforementioned transformations.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form): Move warnings...
* lisp/emacs-lisp/cconv.el (cconv-convert): ...here, which is an
overall better place (closer to the front-end).
Rewrite (prog1 CONST FORMS...) => (progn FORMS... CONST)
where CONST is a compile-time constant, because putting the value last
allows the lapcode peephole pass to do important improvements like
branch elimination. Also use progn instead of prog1 for `ignore`.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
New `prog1` and `ignore` transforms.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Remove for-effect uses of lexical variables. We previously relied on
this being done by the lapcode peephole optimiser but at source level
it enables more optimisation opportunities.
Keywords are elided for the same reason.
Make the optimiser aware of lexical arguments. Otherwise we cannot
know for sure whether a variable is lexical or dynamic during
traversal.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-one-form): New optimiser
entry point, replacing the recursive byte-optimize-form.
* lisp/emacs-lisp/bytecomp.el (byte-optimize-one-form): Autoload.
(byte-compile-keep-pending, byte-compile-top-level):
Use byte-optimize-one-form.
Wrong number of arguments in inlining function calls (to `defsubst` or
explicitly using `inline`) did not result in warnings, or in very
cryptic ones.
* lisp/emacs-lisp/byte-opt.el (byte-compile-inline-expand): Add calls
to `byte-compile--check-arity-bytecode`.
* lisp/emacs-lisp/bytecomp.el (byte-compile-emit-callargs-warn)
(byte-compile--check-arity-bytecode): New functions.
(byte-compile-callargs-warn): Use factored-out function.
* test/lisp/emacs-lisp/bytecomp-resources/warn-callargs-defsubst.el:
* test/lisp/emacs-lisp/bytecomp-tests.el ("warn-callargs-defsubst.el"):
New test case.
Found by Pip Cet.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-quote): Fix mistake that
made this optimiser ineffective at removing quoting of nil, t, and
keywords. The only obvious consequence is that we no longer need...
(byte-optimize-form): ...a 'nil => nil normalising step here; remove.
(byte-optimize-form-code-walker): Make the compiler warn about (quote).
Functions compiled when inlined (thus from inside the optimiser)
mustn't retain the lexical environment of the caller or there will be
tears. See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01227.html .
Bug found by Stefan Monnier.
* lisp/emacs-lisp/byte-opt.el (byte-compile-inline-expand):
Bind byte-optimize--lexvars to nil when re-entering the compiler
recursively.
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el:
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el: New files.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-defsubst): New test.
(byte-compile-inline-expand): When inlining code from another file,
always inline the byte-code version of the function.
(byte-optimize--pcase): Simplify edebug spec.
It's good practice to compare integers using 'eql' because two bignum
objects representing the same integer might not be 'eq'. However,
'eql' is slower and doesn't have its own byte code. Therefore,
replace it with 'eq' if one argument is guaranteed to be a fixnum on
all platforms.
* lisp/emacs-lisp/byte-opt.el (byte-optimize--fixnump): New helper
function.
(byte-optimize-equal, byte-optimize-member, byte-optimize-assoc): Use
it to optimize 'eql' etc. to 'eq' if it will always compare fixnums.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker): Don't
perform incorrect optimisations when a condition-case variable shadows
another lexical variable.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
New test case.
* lisp/emacs-lisp/byte-opt.el (pure-fns): Treat string>,
string-greaterp, string-empty-p, string-blank-p, string-prefix-p and
string-suffix-p as pure functions in the compiler.
* doc/lispref/functions.texi (Calling Functions): Document it.
* lisp/subr.el (always): New function.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Mark it as
side effect free.
There is no point in traversing conditional branches that are
statically known never to be executed. This saves some optimisation
effort, but more importantly prevents variable assignments and
references in those branches from blocking effective constant
propagation.
Also attempt to traverse as much as possible in an unconditional
context, which enables constant-propagation through (linear)
assignments.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form):
Rewrite the (tail) recursion into an explicit loop. Normalise a
return value of (quote nil) to nil, for easier subsequent
optimisations.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker): Don't
traverse dead `if` branches. Use unconditional traversion context
when possible.
This bug was introduced by the lexical variable constant propagation
mechanism. It was discovered by Michael Heerdegen.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form)
(byte-optimize-body): Let the effects of a local defvar declaration be
scoped by let and let*, not any arbitrary Lisp expression body (such
as progn).
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--get-vars)
(bytecomp-local-defvar): New test.
The recent change in macroexp triggered a cyclic dependency error
during eager macroexpansion when neither `bytecomp` nor `byte-opt` had
been byte-compiled yet. This fixes it by moving the offending
function to macroexp.el.
* lisp/emacs-lisp/macroexp.el (macroexp--unfold-lambda): Move from
byte-opt.el and rename.
(macroexp--expand-all): Use it.
* lisp/emacs-lisp/byte-opt.el (byte-compile-unfold-lambda): Move to
macroexp.el.
(byte-compile-inline-expand, byte-optimize-form-code-walker):
* lisp/emacs-lisp/bytecomp.el (byte-compile-form):
Use `macroexp--unfold-lambda` instead.
* lisp/emacs-lisp/byte-opt.el (byte-optimize--lexvars)
(byte-optimize--vars-outside-condition)
(byte-optimize-form-code-walker, byte-optimize-let-form):
Clarify various aspects in the variable constant-propagation code,
as kindly pointed out by Stefan Monnier.
Lexical variables bound to a constant value (symbol, number or string)
are substituted at their point of use and the variable then eliminated
if possible. Example:
(let ((x (+ 2 3))) (f x)) => (f 5)
This reduces code size, eliminates stack operations, and enables
further optimisations. The implementation is conservative, and is
strongly curtailed by the presence of variable mutation, conditions
and loops.
* lisp/emacs-lisp/byte-opt.el
(byte-optimize-enable-variable-constprop)
(byte-optimize-warn-eliminated-variable): New constants.
(byte-optimize--lexvars, byte-optimize--vars-outside-condition)
(byte-optimize--vars-outside-loop, byte-optimize--dynamic-vars):
New dynamic variables.
(byte-optimize--substitutable-p, byte-optimize-let-form):
New functions.
(byte-optimize-form-code-walker): Adapt clauses for variable
constprop, and add clauses for 'setq' and 'defvar'.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-test-var)
(bytecomp-test-get-var, bytecomp-test-identity)
(byte-opt-testsuite-arith-data): Add test cases.
Before this patch doing:
rm lisp/calendar/calendar.elc
make lisp/calendar/cal-hebrew.elc
would spew out lots of spurious such warnings about a `date` argument,
pointing to code which has no `date` argument in sight. This was
because that code had calls to inlinable functions (taking a `date`
argument) defined in `calendar.el`, and while `date` is a normal
lexical var at the site of those functions' definitions, it was
declared as dynbound at the call site.
* lisp/emacs-lisp/byte-opt.el (byte-compile-inline-expand):
Don't impose our local context onto the inlined function.
* test/lisp/emacs-lisp/bytecomp-tests.el: Add matching test.
This introduces two new optimizations. They're designed for code like
(while
(let (...)
(if ... (progn blabla t) (progn blabla nil)))
...)
and they allow the elimination of the test internal to `while` since
we can immediately know when we return `t` or `nil` what the result
of the test will be.
`cl-labels` tends to generate this kind of code when it applies the
tail-call optimization.
This moves two optimizations from the final pass to the main loop.
Both may enable further optimizations (and the second can be applied
repeatedly but "from the end", so the loop in the final pass only gets
to apply it once).
* doc/lispref/sequences.texi (Sequence Functions): Document them.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Mark them as
side-effect-free.
* lisp/emacs-lisp/shortdoc.el (list): Mention them.
* src/fns.c (Flength): Mention them in the doc string.
(length_internal): New function.
(Flength_less, Flength_greater, Flength_equal): New defuns.
(syms_of_fns): Sym them.
