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BUILD: Replace _need_memalign runtime test by hardcoded list

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According to a discussion on -devel, this test cannot work reliably in
general: It cannot determine when unaligned access really works reliably
in all situations, nor on all implementations of the target CPU arch;
nor does it determine whether unaligned access is supported effectively
(as opposed to say supported via super-slow fault handler mechanism).
This commit is contained in:
Max Horn 2011-05-06 13:14:44 +02:00
parent 4b7f6dfa3c
commit d2e778bf0b
1 changed files with 20 additions and 34 deletions
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54
configure vendored
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@ -2061,48 +2061,34 @@ fi
# alignment can be a lot slower than regular access, so we don't want
# to use it if we don't have to.
#
# So we do the following: First, for CPU families where we know whether
# unaligned access is safe & fast, we enable / disable unaligned access
# accordingly.
# Otherwise, for cross compiled builds we just disable memory alignment.
# For native builds, we run some test code that detects whether unaligned
# access is supported (and is supported without an exception handler).
# So we do the following: For CPU families where we know whether unaligned
# access is safe & fast, we enable / disable unaligned access accordingly.
# Otherwise, we just disable memory alignment.
#
# NOTE: The only kinds of unaligned access we allow are for 2 byte and
# 4 byte loads / stores. No promises are made for bigger sizes, such as
# 8 or 16 byte loads, for which various architectures (e.g. x86 and PowerPC)
# behave differently than for the smaller sizes).
# NOTE: In the past, for non-cross compiled builds, we would also run some code
# which would try to test whether unaligned access worked or not. But this test
# could not reliably determine whether unaligned access really worked in all
# situations (and across different implementations of the target CPU arch), nor
# whether it was fast (as opposed to slowly emulated by fault handlers). Hence,
# we do not use this approach anymore.
#
# NOTE: The only kinds of unaligned access we allow are for 2 byte and 4
# byte loads / stores. No promises are made for bigger sizes, such as 8
# or 16 byte loads, for which architectures may behave differently than
# for the smaller sizes.
echo_n "Alignment required... "
case $_host_cpu in
i[3-6]86 | x86_64 | ppc*)
# Unaligned access should work
_need_memalign=no
;;
alpha* | arm* | bfin* | hp* | mips* | sh* | sparc* | ia64 | nv1*)
# Unaligned access is not supported or extremely slow.
_need_memalign=yes
;;
i[3-6]86 | x86_64 | ppc*)
# Unaligned access should work reasonably well
_need_memalign=no
;;
*)
if test -z "$_host"; then
# NOT in cross-compiling mode:
# Try to auto-detect....
cat > $TMPC << EOF
#include <stdlib.h>
#include <signal.h>
int main(int argc, char **argv) {
unsigned char test[8] = { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88 };
signal(SIGBUS, exit);
signal(SIGABRT, exit);
signal(SIGSEGV, exit);
if (*((unsigned int *)(test + 1)) != 0x55443322 && *((unsigned int *)(test + 1)) != 0x22334455) {
return 1;
}
return 0;
}
EOF
cc_check_no_clean && $TMPO$HOSTEXEEXT && _need_memalign=no
cc_check_clean
fi
# Status of unaligned access is unknown, so assume the worst.
_need_memalign=yes
;;
esac
echo "$_need_memalign"