Module Name: src
Committed By: rillig
Date: Sat Sep 4 21:20:44 UTC 2021
Modified Files:
src/tests/usr.bin/xlint/lint1: msg_117.c msg_117.exp
Log Message:
tests/lint: provide more background information on signed '>>'
To generate a diff of this commit:
cvs rdiff -u -r1.9 -r1.10 src/tests/usr.bin/xlint/lint1/msg_117.c
cvs rdiff -u -r1.8 -r1.9 src/tests/usr.bin/xlint/lint1/msg_117.exp
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/tests/usr.bin/xlint/lint1/msg_117.c
diff -u src/tests/usr.bin/xlint/lint1/msg_117.c:1.9 src/tests/usr.bin/xlint/lint1/msg_117.c:1.10
--- src/tests/usr.bin/xlint/lint1/msg_117.c:1.9 Fri Aug 27 17:59:46 2021
+++ src/tests/usr.bin/xlint/lint1/msg_117.c Sat Sep 4 21:20:44 2021
@@ -1,4 +1,4 @@
-/* $NetBSD: msg_117.c,v 1.9 2021/08/27 17:59:46 rillig Exp $ */
+/* $NetBSD: msg_117.c,v 1.10 2021/09/04 21:20:44 rillig Exp $ */
# 3 "msg_117.c"
// Test for message: bitwise '%s' on signed value possibly nonportable [117]
@@ -66,3 +66,70 @@ shr_unsigned_char_promoted_unsigned(unsi
*/
return (unsigned char)((unsigned char)(bit - 1) >> 5);
}
+
+/*
+ * C90 3.3.7, C99 6.5.7 and C11 6.5.7 all say the same: If E1 has a signed
+ * type and a negative value, the resulting value is implementation-defined.
+ *
+ * These standards don't guarantee anything about the lower bits of the
+ * resulting value, which are generally independent of whether the shift is
+ * performed in signed arithmetics or in unsigned arithmetics. The C99
+ * rationale talks about signed shifts, but does not provide any guarantee
+ * either. It merely suggests that platforms are free to use unsigned shifts
+ * even if the operand type is signed.
+ *
+ * K&R provides more guarantees by saying: Right shifting a signed quantity
+ * will fill with sign bits ("arithmetic shift") on some machines such as the
+ * PDP-Il, and with 0-bits ("logical shift") on others.
+ *
+ * https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html says:
+ * Signed '>>' acts on negative numbers by sign extension.
+ *
+ * This means that at least in GCC mode, lint may decide to not warn about
+ * these cases.
+ */
+void
+shr_signed_ignoring_high_bits(int x)
+{
+
+ /*
+ * All sane platforms should define that 'x >> 0 == x', even if x is
+ * negative.
+ */
+ /* expect+1: warning: bitwise '>>' on signed value possibly nonportable [117] */
+ if (x >> 0 != 0)
+ return;
+
+ /*
+ * If x is negative, x >> 1 is nonzero, no matter whether the shift
+ * is arithmetic or logical.
+ */
+ /* expect+1: warning: bitwise '>>' on signed value possibly nonportable [117] */
+ if (x >> 1 != 0)
+ return;
+
+ /*
+ * The highest bit may be 0 or 1, the others should be well-defined
+ * on all sane platforms, making it irrelevant whether the actual
+ * shift operation is arithmetic or logical.
+ */
+ /* expect+1: warning: bitwise '>>' on signed value possibly nonportable [117] */
+ if (((x >> 1) & 1) != 0)
+ return;
+
+ /*
+ * The result of this expression is the same with arithmetic and
+ * logical shifts since the filled bits are masked out.
+ */
+ /* expect+1: warning: bitwise '>>' on signed value possibly nonportable [117] */
+ if (((x >> 31) & 1) != 0)
+ return;
+
+ /*
+ * In this case, arithmetic shift results in 2 while logical shift
+ * results in 0. This difference is what this warning is about.
+ */
+ /* expect+1: warning: bitwise '>>' on signed value possibly nonportable [117] */
+ if (((x >> 31) & 2) != 0)
+ return;
+}
Index: src/tests/usr.bin/xlint/lint1/msg_117.exp
diff -u src/tests/usr.bin/xlint/lint1/msg_117.exp:1.8 src/tests/usr.bin/xlint/lint1/msg_117.exp:1.9
--- src/tests/usr.bin/xlint/lint1/msg_117.exp:1.8 Fri Aug 27 17:59:46 2021
+++ src/tests/usr.bin/xlint/lint1/msg_117.exp Sat Sep 4 21:20:44 2021
@@ -5,3 +5,8 @@ msg_117.c(29): warning: shift amount 466
msg_117.c(35): warning: bitwise '>>' on signed value possibly nonportable [117]
msg_117.c(35): warning: negative shift [121]
msg_117.c(57): warning: bitwise '>>' on signed value possibly nonportable [117]
+msg_117.c(100): warning: bitwise '>>' on signed value possibly nonportable [117]
+msg_117.c(108): warning: bitwise '>>' on signed value possibly nonportable [117]
+msg_117.c(117): warning: bitwise '>>' on signed value possibly nonportable [117]
+msg_117.c(125): warning: bitwise '>>' on signed value possibly nonportable [117]
+msg_117.c(133): warning: bitwise '>>' on signed value possibly nonportable [117]
