29415 2009-09-18 07:36:34 -0700 Byte alignment issue on MIPS 2010-08-27 10:02:50 -0700 1 1 1 Unclassified WebKit Tools / Tests 528+ (Nightly build) Other Other RESOLVED FIXED P2 Normal --- 1 vestbo webkit-unassigned abarth commit-queue darin diegohcg eric hausmann kent.hansen morrita plaes tonikitoo webkit.review.bot oldest_to_newest 148120 0 vestbo 2009-09-18 07:36:34 -0700 This bug report originated from issue QTBUG-3991 <http://bugreports.qt.nokia.com/browse/QTBUG-3991> --- Description --- We have found an issue in the following function: static inline bool equal(StringImpl* string, const UChar* characters, unsigned length) in AtomicString.cpp. The issue is that the variables 'stringCharacters' and 'bufferCharacters' are uint32 pointers but in fact point to UChar* variables. This means that the starting addresses might be not aligned to 4 bytes. On MIPS an unaligned exception is raised and in our case is handled by the kernel, but, depending on the kernel, it can also lead to the kernel sending SIGBUS to the application. The kernel exception handling is very costly and below is a solution to this problem; basically we define a structure with the 'packed' attribute which will make the compiler to generate code capable of handling unaligned accesses (on MIPS this means replacing 'lw' with a pair: 'lwl' and 'lwr'). I saw that Q_PACKED is defined only for the GCC and Intel compiler which should make our change safe for all cases. Another solution is to simply add || PLATFORM(MIPS) where the check for ARM is made. Patch: --- a/src/3rdparty/webkit/WebCore/platform/text/AtomicString.cpp +++ b/src/3rdparty/webkit/WebCore/platform/text/AtomicString.cpp @@ -96,6 +96,10 @@ struct UCharBuffer { unsigned length; }; +struct unaligned_struct { + uint32_t i; +} Q_PACKED; + static inline bool equal(StringImpl* string, const UChar* characters, unsigned length) { if (string->length() != length) @@ -111,17 +115,21 @@ static inline bool equal(StringImpl* string, const UChar* characters, unsigned l #else /* Do it 4-bytes-at-a-time on architectures where it's safe */ - const uint32_t* stringCharacters = reinterpret_cast<const uint32_t*>(string->characters()); - const uint32_t* bufferCharacters = reinterpret_cast<const uint32_t*>(characters); + const struct unaligned_struct *stringCharacters = + reinterpret_cast<const struct unaligned_struct*>(string->characters()); + const struct unaligned_struct *bufferCharacters = + reinterpret_cast<const struct unaligned_struct*>(characters); unsigned halfLength = length >> 1; - for (unsigned i = 0; i != halfLength; ++i) { - if (*stringCharacters++ != *bufferCharacters++) + for (unsigned i = 0; i != halfLength; ++i, stringCharacters++, bufferCharacters++) { + if (stringCharacters->i != bufferCharacters->i) return false; } - if (length & 1 && *reinterpret_cast<const uint16_t*>(stringCharacters) != *reinterpret_cast<const uint16_t*>(bufferCharacters)) + if (length & 1 && + *reinterpret_cast<const uint16_t*>(stringCharacters) != *reinterpret_cast<const uint16_t*>(bufferCharacters)) { return false; + } return true; #endif 200371 1 diegohcg 2010-03-16 12:35:32 -0700 The OS should be Other in this bug 200687 2 kent.hansen 2010-03-17 04:49:36 -0700 This is not a Qt-specific issue. +1 for adding CPU(MIPS) to the "going 4 bytes at a time is unsafe" condition, though. 203869 3 51602 plaes 2010-03-25 00:34:49 -0700 Created attachment 51602 Aligment patch from Gentoo for ARM, SPARC and SH4 platforms This is the patch we are using on Gentoo to fix alignment issues on SPARC and SH4 platforms. 221566 4 hausmann 2010-05-06 01:38:36 -0700 Removing Qt keyword and Qt from the title, this is not a Qt specific issue. 221567 5 hausmann 2010-05-06 01:39:46 -0700 (In reply to comment #3) > Created an attachment (id=51602) [details] > Aligment patch from Gentoo for ARM, SPARC and SH4 platforms > > This is the patch we are using on Gentoo to fix alignment issues on SPARC and > SH4 platforms. Priit, can you submit your patch with ChangeLog, diffed against tip of tree and with review? set? See also http://webkit.org/coding/contributing.html Thanks :) 260983 6 63668 fu 2010-08-05 17:34:17 -0700 Created attachment 63668 Access one byte at a time for MIPS To fix this issue, we follow ARM and SH4 to access one byte at a time. This improves the performance on MIPS, due to no kernel involvement to fix unaligned accesses. In the future, we still can use lwl/lwr to improve the performance. Thanks! Regards, Chao-ying 263563 7 morrita 2010-08-11 18:37:43 -0700 Look really a trivial and small change. Anyone who has MIPS experience is looks good candidate for just giving r+. 270212 8 63668 oliver 2010-08-26 16:51:46 -0700 Comment on attachment 63668 Access one byte at a time for MIPS r=me 270425 9 63668 commit-queue 2010-08-27 05:00:52 -0700 Comment on attachment 63668 Access one byte at a time for MIPS Clearing flags on attachment: 63668 Committed r66205: <http://trac.webkit.org/changeset/66205> 270426 10 commit-queue 2010-08-27 05:00:58 -0700 All reviewed patches have been landed. Closing bug. 270450 11 webkit.review.bot 2010-08-27 05:59:47 -0700 http://trac.webkit.org/changeset/66205 might have broken Qt Linux Release 270565 12 darin 2010-08-27 10:02:50 -0700 Maybe we should reverse this check. Maintaining list of platforms that allow unaligned access might be better than a list of platforms that do not. 51602 2010-03-25 00:34:49 -0700 2010-06-11 11:59:22 -0700 Aligment patch from Gentoo for ARM, SPARC and SH4 platforms webkit-gtk-1.1.15.2-unaligned.patch text/plain 4403 plaes LS0tIHdlYmtpdC0xLjEuMTYub3JpZy9KYXZhU2NyaXB0Q29yZS93dGYvUGxhdGZvcm0uaAorKysg d2Via2l0LTEuMS4xNi9KYXZhU2NyaXB0Q29yZS93dGYvUGxhdGZvcm0uaApAQCAtMzQ3LDYgKzM0 NywyMyBAQAogI2RlZmluZSBXVEZfUExBVEZPUk1fWDg2XzY0IDEKICNlbmRpZgogCisvKiBQTEFU Rk9STShJQTY0KSAqLworI2lmIGRlZmluZWQoX19pYTY0X18pCisjZGVmaW5lIFdURl9QTEFURk9S TV9JQTY0IDEKKyNlbmRpZgorCisvKiBQTEFURk9STShBTFBIQSkgKi8KKyNpZiBkZWZpbmVkKF9f YWxwaGFfXykKKyNkZWZpbmUgV1RGX1BMQVRGT1JNX0FMUEhBIDEKKyNlbmRpZgorCisvKiBQTEFU Rk9STShTUEFSQykgKi8KKyNpZiAgIGRlZmluZWQoX19zcGFyY19fKSBcCisgICB8fCBkZWZpbmVk KF9fc3BhcmMpCisjZGVmaW5lIFdURl9QTEFURk9STV9TUEFSQyAxCisjZGVmaW5lIFdURl9QTEFU Rk9STV9CSUdfRU5ESUFOIDEKKyNlbmRpZgorCiAvKiBQTEFURk9STShTSDQpICovCiAjaWYgZGVm 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