diff mbox

[3/5] Disable warn about left shifts of negative values

Message ID 20160809190229.27871-3-bobby.prani@gmail.com (mailing list archive)
State New, archived
Headers show

Commit Message

Pranith Kumar Aug. 9, 2016, 7:02 p.m. UTC 
It seems like there's no good reason for the compiler to exploit the
undefinedness of left shifts.  GCC explicitly documents that they do not
use at all this possibility and, while they also say this is subject
to change, they have been saying this for 10 years (since the wording
appeared in the GCC 4.0 manual).

Disable these warnings by passing in -Wno-shift-negative-value.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[pranith: forward-port part of patch to 2.7]
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 HACKING   | 4 ++++
 configure | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)
diff mbox

Patch

diff --git a/HACKING b/HACKING
index 058aa8f..20a9101 100644
--- a/HACKING
+++ b/HACKING
@@ -158,6 +158,10 @@  painful. These are:
  * you may assume that right shift of a signed integer duplicates
    the sign bit (ie it is an arithmetic shift, not a logical shift)
 
+In addition, QEMU assumes that the compiler does not use the latitude
+given in C99 and C11 to treat aspects of signed '<<' as undefined, as
+documented in the GNU Compiler Collection manual starting at version 4.0.
+
 7. Error handling and reporting
 
 7.1 Reporting errors to the human user
diff --git a/configure b/configure
index f57fcc6..8d84919 100755
--- a/configure
+++ b/configure
@@ -1452,7 +1452,7 @@  fi
 gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
 gcc_flags="-Wmissing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
-gcc_flags="-Wendif-labels $gcc_flags"
+gcc_flags="-Wendif-labels -Wno-shift-negative-value $gcc_flags"
 gcc_flags="-Wno-initializer-overrides $gcc_flags"
 gcc_flags="-Wno-string-plus-int $gcc_flags"
 # Note that we do not add -Werror to gcc_flags here, because that would