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use and to test the 21st bit

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gnzlbg 2018-06-24 19:20:53 +02:00 • committed by gnzlbg
parent d6290b5e88
commit 7affba30a5
1 changed files with 18 additions and 13 deletions
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31
library/stdarch/coresimd/x86/cpuid.rs
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@ -96,31 +96,36 @@ pub fn has_cpuid() -> bool {
let result: u32;
let _temp: u32;
asm!(r#"
# Read eflags into $0 and copy into $1:
# Read eflags into $0 and copy it into $1:
pushfd
pop $0
mov $1, $0
# Flip 21st bit:
# Flip 21st bit of $0.
xor $0, 0x200000
# Set eflags:
# Set eflags to the value of $0
#
# Bit 21st can only be modified if cpuid is available
push $0
popfd
# Read eflags again, if cpuid is available
# the 21st bit will be flipped, otherwise it
# it will have the same value as the original in $1:
pushfd
popfd # A
# Read eflags into $0:
pushfd # B
pop $0
# Xor'ing with the original eflags should have the
# 21st bit set to true if cpuid is available and zero
# otherwise. All other bits have not been modified and
# are zero:
# xor with the original eflags sets the bits that
# have been modified:
xor $0, $1
# There is a race between popfd (A) and pushfd (B)
# where other bits beyond 21st may have been modified due to
# interrupts, a debugger stepping through the asm, etc.
#
# Therefore, explicitly check whether the 21st bit
# was modified or not:
and $0, 0x200000
"#
: "=r"(result), "=r"(_temp)
:
: "cc", "memory"
: "intel");
// Therefore, if result is 0, the bit was not modified and cpuid is
// If result == 0, the 21st bit was not modified and cpuid is
// not available. If cpuid is available, the bit was modified and
// result != 0.
result != 0