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.
