John Regehr
C and C++ people: I'm looking for an example -- for a target the supports unaligned memory operations -- where code that violates language-level memory alignment requirements is *not* compiled into something that works as one would naively expect.
I vaguely remember people (almost certainly including @void_friend) discussing this some time ago, but I can't find the posts, argh
Jeff "never puts away anything, especially oven mitts" Cliff, Bringer of Nightmares 🏴☠️🦝🐙 🇱🇧🧯 🇨🇦🐧
John Regehr
@jripley yeah! one of the blog posts I linked is that exact scenario
John Marshall
@regehr @jripley Our genomics library suffered from this on x86-64 a while back: data structures with unaligned uint32_ts crashed when user code was compiled with vectorisation. It was a shame that these uint32_t CIGAR entries weren't aligned in the first place, which I've long used as an example of the perils of bioinformaticians single-handedly designing formats without any input from anyone with a CS background… #ThisIsBioinformatics
John Ripley
@regehr There's an insidious problem on targets where the ISA has a few instructions which rely on alignment (e.g some SSE/AVX and even recent ARM). Unaligned access works until the optimizer figures out a way to save a cycle by using the faster vector op, and then it breaks. This turns into a heisenbug where any attempt to track it down breaks the optimization.
Edwin Török
@johnm @regehr Another example. testcase that (used to) breaks with
GCC -O3 on x86-64 https://github.com/Cisco-Talos/clamav/commit/c47e43ce354878de55f3521c9b8fbc887ed206f4
Attempted fix https://github.com/Cisco-Talos/clamav/commit/2b481467801751d79d1bcd40878c070e7bc03bcb
This was from ~17 years ago, and TBH I'm not convinced the "fix" avoids UB completely, but I assume someone would've found a unit test failure by now, but I see the code still looks the same now. Perhaps the only way to really avoid UB there is to use `memcpy`, and hope the compiler will be smart enough to optimize it away.
Edwin Török
@regehr @void_friend LLVM bug 6910: malloc/new only returns memory aligned to 8 bytes, but GCC would assume a 16-byte alignment.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.