[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH v5 01/46] include: Add IEC binary prefixes in "q

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v5 01/46] include: Add IEC binary prefixes in "qemu/units.h"
Date: Fri, 29 Jun 2018 07:19:16 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

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).


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

reply via email to

[Prev in Thread] Current Thread [Next in Thread]