https://bugs.kde.org/show_bug.cgi?id=367543
Bug ID: 367543
Summary: bt/btc/btr/bts x86/x86_64 instructions are
poorly-handled wrt flags
Product: valgrind
Version: 3.10.0
Platform: Other
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: vex
Assignee: jsew...@acm.org
Reporter: shortc...@fastmail.com
According to the code, O,S,Z,A,P are being forced to 0, under the rationale
that they are "undefined", with no (apparent) user-visible warnings emitted
regarding
the effects this has on future instructions with dependencies on these flags,
which is less than ideal.
Additionally, the Z flag should be unmodified, not undefined, per real-world
CPU behavior and this excerpt(for BT) from Intel's June 2016 architecture
manual:
"The CF flag contains the value of the selected bit. The ZF flag is unaffected.
The OF, SF, AF, and PF flags are undefined."
Quick and dirty test case derived from a larger program:
int main(int argc, char* argv[])
{
unsigned a = 0;
unsigned b = 1;
asm volatile(
"testl $15, %%eax\n\t"
"bt $15, %%ebx\n\t"
"cmovbe %%ebx, %%eax\n\t"
: "=a"(a)
: "a"(a), "b"(b)
: "cc" );
__builtin_printf("%u\n", a);
if(a != 1)
__builtin_abort();
return 0;
}
Reproducible: Always
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