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cc: fix wrong "useless or misleading comparison" warning

Browse source 
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.
This commit is contained in:
cinap_lenrek 2018-11-18 20:42:45 +01:00
parent b6251bff91
commit 485a3301e6
1 changed files with 17 additions and 10 deletions
Download patch file Download diff file Expand all files Collapse all files

27
sys/src/cmd/cc/com.c
View file

@ -1373,14 +1373,21 @@ compar(Node *n, int reverse)
/*
* Skip over left casts to find out the original expression range.
*/
while(l->op == OCAST)
while(l->op == OCAST){
lt = l->type;
rt = l->left->type;
if(lt == T || rt == T)
return 0;
if(lt->width < rt->width)
break;
if(lt->width == rt->width && ((lt->etype ^ rt->etype) & 1) != 0)
break;
l = l->left;
}
if(l->op == OCONST)
return 0;
lt = l->type;
if(lt == T)
return 0;
if(lt->etype == TXXX || lt->etype > TUVLONG)
if(lt == T || lt->etype == TXXX || lt->etype > TUVLONG)
return 0;
/*
@ -1399,16 +1406,17 @@ compar(Node *n, int reverse)
if((rt->etype&1) && r->vconst < 0) /* signed negative */
x.a = ~0ULL;
if((lt->etype&1)==0){
if(lt->etype & 1){
/* signed */
lo = big(~0ULL, -(1LL<<(lt->width*8-1)));
hi = big(0, (1LL<<(lt->width*8-1))-1);
} else {
/* unsigned */
lo = big(0, 0);
if(lt->width == 8)
hi = big(0, ~0ULL);
else
hi = big(0, (1LL<<(l->type->width*8))-1);
}else{
lo = big(~0ULL, -(1LL<<(l->type->width*8-1)));
hi = big(0, (1LL<<(l->type->width*8-1))-1);
hi = big(0, (1LL<<(lt->width*8))-1);
}
switch(op){
@ -1462,4 +1470,3 @@ if(debug['y']) prtree(n, "strange");
warn(n, "useless or misleading comparison: %s", cmpbuf);
return 0;
}