From: Steven Rostedt
Sent: 16 December 2022 17:38
On Fri, 16 Dec 2022 12:19:47 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
I assumed that "memory" was for memory unrelated to the input constraints.
Well, it looks like you do need a "memory" barrier.
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
"memory"
The "memory" clobber tells the compiler that the assembly code
performs memory reads or writes to items other than those listed in
the input and output operands (for example, accessing the memory
pointed to by one of the input parameters). To ensure memory contains
correct values, GCC may need to flush specific register values to
memory before executing the asm. Further, the compiler does not
assume that any values read from memory before an asm remain
unchanged after that asm; it reloads them as needed. Using the
"memory" clobber effectively forms a read/write memory barrier for
the compiler.
As the "(for example, accessing the memory pointed to by one of the input
parameters)" is exactly this case.
Without the memory clobber code like:
int f(const char *s)
{
char c[4] = "abc";
return strcmp(s, c);
}
is very like to get optimised so that c[] is never written.
However, in this case, the strings have all existed for ages.
So that won't be the problem.
It might be obvious what is wrong from the asm output.
Although the binary-chop lookup is suspect I'd also check
that the sorted index is plausible - just tracing the first
20 entries might be enough.
No point peering at the search code if the setup is wrong.
David
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