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configure: Always compile with -fwrapv

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QEMU's code relies on left shifts of signed integers always
being defined behaviour with the obvious 2s-complement
semantics. The only way to tell the compiler (and any
associated undefined-behaviour sanitizer) that we require a
C dialect with these semantics is to use the -fwrapv option.
This is a bit of a heavy hammer for the job as it also gives
us guaranteed semantics on integer arithmetic overflow which
in theory we don't require.

In an ideal world this would allow us to drop the warning
flag -Wno-shift-negative-value, but we must retain this to
avoid spurious warnings on clang versions predating the
fix to https://llvm.org/bugs/show_bug.cgi?id=25552.

Backports commit 2d31515bc0880a1cea86ce638d2a109f4f4e6f7d from qemu
This commit is contained in:
Peter Maydell 2018-02-25 23:17:33 -05:00 committed by Lioncash
parent 8b5400d675
commit eb77f61bea
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GPG key ID: 4E3C3CC1031BA9C7
1 changed files with 5 additions and 1 deletions
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6
qemu/configure vendored
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@ -223,7 +223,11 @@ strip="${STRIP-${cross_prefix}strip}"
ARFLAGS="${ARFLAGS-rv}" ARFLAGS="${ARFLAGS-rv}"
# default flags for all hosts # default flags for all hosts
QEMU_CFLAGS="-fno-strict-aliasing -fno-common $QEMU_CFLAGS" # We use -fwrapv to tell the compiler that we require a C dialect where
# left shift of signed integers is well defined and has the expected
# 2s-complement style results. (Both clang and gcc agree that it
# provides these semantics.)
QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS" QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS" QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS" QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"