ccom may be called multiple times on the same
node, via 'goto loop' calls from the commute
label, OADD, and a few other places.
Casts to void could null out the LHS of the
node, which would cause the compiler to crash
if the cast was revisited due to one of these
cases, because we tried frobbing n->left.
Now, if n->left is nil, we just return.w
Section 6.5.15 of the C99 spec requires that if
one argument of a ?: expression is a null pointer
constant, and the other has a pointer type T*, then
the type of the expression is T*.
We were attempting to follow this rule, however,
we only handled literal expressions when checking
for null pointers.
This change looks through casts, so 'nil' and 'NULL',
and their expansion '(void*)0' are all detected as
null pointer constants.
the following code reproduces the crash:
void
foo(void)
{
}
void
main(int argc, char **argv)
{
(void)(1 ? (void)0 : foo());
}
the problem is that side() gives a false positive on the OCOND
with later constant folding eleminating the acutal side effect
and OCAST ending up with two nested OCATS with the nested one
being zapped (type == T).
implicit casts would cause spurious "result of operation not used"
warnings such as ape's stdio putc() macro.
make (void) casts non-ops when the casted expression has no
side effects. this avoid spurious warning with ape's assert()
macro.
This change imports a few warnings and minor fixes from Charles branch
here: https://bitbucket.org/plan9-from-bell-labs/plan9.
The changes included here:
changeset: 1374:9185dc017be0
summary: declare castucom; move a declaration into order;
use cast instead of ULL suffix
changeset: 1353:5fe8380b1818
summary: supporting functions:
1. castucom to match unlikely mask operation;
2. be sure to snap both sides of pointer subtraction completely;
3. add extra operators as side-effect free
changeset: 1352:90058c092d66
summary: 1. correct result type for mixed-mode assignment operators
2. detect divide by zero (erik);
3. detect masks misformed by sign-extension;
4. diagnose mixed old/new prototypes
to reproduce:
u8int x, y;
x = 0xff;
y = 0xc0;
if((s8int)(x & y) >= 0)
print("help\n");
compiles correctly but prints a warning
warning: test.c:11 useless or misleading comparison: UINT >= 0
the issue is that compar() unconditionally skipped over
all left casts ignoring the case when a cast would sign
extend the value.
the new code only skips over the cast when the original
type with is smaller than the cast result or when they
are equal width and types have same signedness. so the
effective left hand side type is the last truncation
or sign extension.
> 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.
