On 06/28/2018 05:53 PM, Philippe Mathieu-Daudé wrote:
+#ifndef QEMU_UNITS_H
+#define QEMU_UNITS_H
+
+#define KiB (INT64_C(1) << 10)
+#define MiB (INT64_C(1) << 20)
+#define GiB (INT64_C(1) << 30)
+#define TiB (INT64_C(1) << 40)
+#define PiB (INT64_C(1) << 50)
+#define EiB (INT64_C(1) << 60)
Shouldn't above use UINT64_C()
Since the decision of signed vs. unsigned was intentional based on
review on earlier versions, it may be worth a comment in this file that
these constants are intentionally signed (in usage patterns, these tend
to be multiplied by another value; and while it is easy to go to
unsigned by doing '1U * KiB', you can't go in the opposite direction if
you want a signed number for '1 * KiB' unless KiB is signed).
OK.
Actually, '1U * KiB' still ends up signed. Why? Because as written, KiB
is a 64-bit quantity, but 1U is 32-bit; type promotion says that since a
64-bit int can represent all 32-bit unsigned values, the result of the
expression is still signed 64-bit.
To get unsigned KiB, you either have to use '1ULL * KiB', or KiB should
be changed to be (INT32_C(1) << 10) (a 32-bit constant, rather than a
64-bit one).
I'll also change this tests using your suggestion:
diff --git a/tests/test-qemu-opts.c b/tests/test-qemu-opts.c
@@ -709,8 +709,7 @@ static void test_opts_parse_size(void)
false, &error_abort);
- g_assert_cmphex(qemu_opt_get_size(opts, "size2", 0),
- ==, 16777215 * T_BYTE);
+ g_assert_cmphex(qemu_opt_get_size(opts, "size2", 0), ==, 16777215U
* TiB);
to avoid this error on 32-bit archs:
source/qemu/tests/test-qemu-opts.c: In function 'test_opts_parse_size':
source/qemu/tests/test-qemu-opts.c:713:71: error: integer overflow in
expression [-Werror=overflow]
g_assert_cmphex(qemu_opt_get_size(opts, "size2", 0), ==, 16777215 * TiB);
^
And this compile error is proof of the confusion when we have a signed
integer overflow (why it only happens on 32-bit arch and not also on
64-bit arch is subtle - it's because TiB is 'long long' on 32-bit, but
merely 'long' on 64-bit, which results in a different type after type
promotion - although I still find it odd that the 64-bit compiler isn't
warning).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
