Hi,
This is patent disinformation. On any decent x86 operating system, the
CPU scheduler that switches between threads preserves all registers and
flags in a per thread context structure somewhere in the OS kernel. The
only thing another thread can do to interfere with a running REP MOVS is
to deallocate / protect the address ranges accessed, and or use low
level thread functions to change the saved registers of a non-running
thread using OS calls such as (on Win32) SetThreadContext(),
TerminateThread() and/or DebugActiveProcess() .
Also, a famous feature of x86 CPUs is that they will happily access
unaligned memory using the basic integer instruction set, which includes
REP MOVSQ, but not some of the wide MMX/SSE instructions that can be
used for optimal memcpy/memmove implementations according to AMD and Intel.
Then there is of cause the ever-present spectre of a buggy CPU not doing
what the code tells it to do. Somewhere else in this thread, someone
mentioned a specific such bug but stated the affected CPU models only in
terms of code names.
On 26/02/2026 22:42, Dimitry Andric via Cygwin wrote:
If such a crash occurs, can you do a "thread apply all bt" in gdb? This will
show what all the other threads are doing. I'm betting some other thread is calling
memcpy or some other function that is messing with the direction flag.
-Dimitry
On 26 Feb 2026, at 21:47, KENNON J CONRAD<[email protected]> wrote:
Yes, lots. 7 threads were running at the point of the crash 87% load on my
i7-4790k. I did a little research since the last post. The memmove code where
the crash occurs is:
0x00007ff96ba812a8 <+136>: std
=> 0x00007ff96ba812a9 <+137>: rep movsq %ds:(%rsi),%es:(%rdi)
0x00007ff96ba812ac <+140>: cld
This sets the direction flag immediately before the rep movsq and clears the
direction flag immediately after the rep movsq. Yet when gdb breaks it shows
the direction flag is not set:
eflags 0x246 [ PF ZF IF ]
Would a forward move on overlapping data cause the SIGTRAP? Could the code
have moved to a different core? Or could it have been interrupted by some
other task that corrupts the flag? As I mentioned earlier, the rep movsq is
only failing once per several million times memmove is called so it seems
likely to be something along those lines.
-Kennon
On 02/26/2026 12:20 PM PST Dimitry Andric<[email protected]> wrote:
Is there some concurrency going on? Maybe some other part of the program is
flipping the direction flag?
-Dimitry
Enjoy
Jakob
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Jakob Bohm, CIO, Partner, WiseMo A/S. https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark. Direct +45 31 13 16 10
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