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optimizers
introduce rolor() function to subsitute (a << c) | (a >> (bits(a) - c))
with (a <<< c) where <<< is cyclic rotation and c is constant.
this almost doubles the speed of chacha encryption of 386 and amd64.
the peephole optimizer used to stop when it hit a shift or rol
instruction when attempting to eleminate moves by register
substitution. but we do not have to as long as the shift count
operand is not CX (which cannot be substituted) and CX is not
a subject for substitution.
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This way the various compiler flags are documented
somewhere, even if not in full.
Also fixed a typo. 'r' should have been 'R'
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from charles forsuth:
because the previous version thought OINDEX might have a side effect, it
stopped it building a tower of them.
probably the best thing is to limit that anyway, since each one consumes
2-3 registers, so towering them can
keep even more active, and the x86 hasn't got that many.
the quick hack is to return that case to the earlier state by treating
OINDEX as a side-effect in side().
it's not a bad thing to do in the OSTRUCT case, for similar reasons: it's
better to collapse the indexed pointer
into a direct register, instead of repeating the indexing operation through
the copying of the value.
OINDEX isn't a machine-independent operation, so it doesn't affect the uses
in ../cc
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this change made 8c fail to build libvorbis and gs, until this
is fixed we can live with non-optimal code.
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- cover more cases that have no side effects
- ensure function has complex FNX
- pull operators out of OFUNC level
- rewrite OSTRUCT lhs to avoid all side-effects, use regalloc() instead of regret()
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conversions
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fixed<->float conversions work correctly
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the compiler used to skip zero initialization when initializer
list was given not covering unspecified elements. now we zero
all non explicitely initialized elements. for example:
typedef struct F F;
struct F
{
int a;
int b;
int c;
};
void
main(void)
{
char a[16] = { 1, 2, 3 }; /* a[3..15] initialized to zero */
F f = { .b = 1 }; /* f.a, f.c initialized to zero */
}
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the emited code that initializes local variables did not handle
unaligned data causing stack corruption, affecting code like:
void main(void)
{
char a[9] = {0};
}
this change will emit code that does byte stores for the unaligned
bytes and also handles small objects (<= 16 bytes) without branches.
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i made a mistake here as this change breaks the arm and mips compilers
which lack an optimiation in xcom() that folds constant pointer arithmetic
into the offset. on arm, the a node is a complex expression with op OADD of
type TIND but the test rejected the (valid) pointer arithmetic.
instead, we now test for the operations which cannot be constant instead
of using the type as a proxy.
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char FOO[] = "abc"; /* ok */
char *BAR = FOO; /* ok */
char *BAZ = BAR; /* wrong */
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(thanks aiju)
> warning: a.c:9 useless or misleading comparison: UINT < 0
the error can be observed by compiling the following code
with warnings enabled:
#include <u.h>
#include <libc.h>
uint r;
void
main(int argc, char *argv[])
{
int r;
if(r < 0){
exits(0);
}
}
the offending code in the compiler is:
- if(l->op == ONAME && l->sym->type){
- lt = l->sym->type;
- if(lt->etype == TARRAY)
- lt = lt->link;
- }
compiler handles scope by overwritin and reverting
symbols while parsing. in the ccom phase, the nodes symbol
(n->sym) is not in the right scope and we wrongly think r
is uint instead of int.
it is not clear to me what this code tried to accomplish in
the first place nor could anyone answer me this question.
the risk is small as this change doesnt affect the compiled
program, only the warning, so removing the offending code.
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we could use 'A' here but then it would require the new acid.
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When running "?c -p ...", ensure the backend cpp recognizes C++ comments.
2c(1) states that the compilers recognize // comments, and the bare compilers
do. But if you invoke the compiler with '-p', the backend cpp process
doesn't handle // comments properly unless you also give ?c the undocumented
'-+' option (which it passes through to cpp).
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