From 485a3301e6119f0c85d7869d469fc0a0888e49c1 Mon Sep 17 00:00:00 2001
From: cinap_lenrek <cinap_lenrek@felloff.net>
Date: Sun, 18 Nov 2018 20:42:45 +0100
Subject: cc: fix wrong "useless or misleading comparison" warning

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.
---
 sys/src/cmd/cc/com.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

(limited to 'sys/src/cmd/cc')

diff --git a/sys/src/cmd/cc/com.c b/sys/src/cmd/cc/com.c
index b8a3b5c44..f67294510 100644
--- a/sys/src/cmd/cc/com.c
+++ b/sys/src/cmd/cc/com.c
@@ -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;
 }
-
-- 
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