0005088: ARCH_INT64_ALIGN incorrectly detected on linux / arm / gcc 4.4.3

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0005088

OCaml

OCaml general

public

2010-06-25 00:56

2012-03-24 15:01

Reporter

jp30

Assigned To

xleroy

Priority

normal

Severity

major

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

3.12.0+beta1 or 3.12.0+rc1

Target Version

Fixed in Version

3.12.0+beta1 or 3.12.0+rc1

Summary

0005088: ARCH_INT64_ALIGN incorrectly detected on linux / arm / gcc 4.4.3

Description

The default behaviour of linux on ARM processors is to ignore alignment faults occurring in user processes (rather than sending a SIGBUS, as is normal on other alignment-sensitive architectures). The instruction causing the fault, however, has undefined results.

Ocaml's configuration script attempts to detect alignment-sensitivity by trying to provoke a SIGBUS in config/auto-aux/int64align.c. On ARM systems running linux, the alignment fault is ignored, leading the configuration script to incorrectly unset ARCH_ALIGN_INT64 in config/m.h.

Steps to reproduce:

$ echo 0 > /proc/cpu/alignment # this is the default - ignore alignment faults
$ ./configure && make world # will SIGSEGV
$ grep ALIGN_INT64 config/m.h
#undef ARCH_ALIGN_INT64
$ echo 4 > /proc/cpu/alignment # alignment faults now cause SIGBUS
$ make clean && make world # will SIGBUS

$ echo 4 > /proc/cpu/alignment # alignment faults cause SIGBUS
$ ./configure && make world # build completes correctly
$ grep ALIGN_INT64 config/m.h
#define ARCH_ALIGN_INT64

Suggested remedy:

Since ignoring alignment faults is default behaviour for linux on ARM, I suggest that the configure script set ARCH_ALIGN_INT64 for all arm*-linux hosts (as is already done for sparc).

The problem doesn't seem to occur with gcc 4.3. Possibly earlier gccs don't provide native 64 bit ints on ARM???

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Notes
(0005580) xleroy (administrator) 2010-07-06 13:33	I'm at a loss to guess which ARM instructions generated by gcc could cause problem with 4-aligned 64-bit accesses. Nonetheless, I updated the configure script as suggested (force alignment for arm--*).

Issue History
Date Modified	Username	Field	Change
2010-06-25 00:56	jp30	New Issue
2010-07-06 13:33	xleroy	Note Added: 0005580
2010-07-06 13:33	xleroy	Assigned To	=> xleroy
2010-07-06 13:33	xleroy	Status	new => resolved
2010-07-06 13:33	xleroy	Resolution	open => fixed
2010-07-06 13:33	xleroy	Fixed in Version	=> 3.12.0+beta1
2012-03-24 15:01	xleroy	Status	resolved => closed