user0/rust
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

fix test and add comments

Browse source
This commit is contained in:
gnzlbg 2018-06-23 15:03:30 +02:00 • committed by gnzlbg
parent 6524ead794
commit 29d969012e
1 changed files with 15 additions and 6 deletions
Download patch file Download diff file Expand all files Collapse all files

21
library/stdarch/coresimd/x86/cpuid.rs
View file

@ -91,22 +91,36 @@ pub fn has_cpuid() -> bool {
// This follows the approach indicated in:
// http://wiki.osdev.org/CPUID#Checking_CPUID_availability
// https://software.intel.com/en-us/articles/using-cpuid-to-detect-the-presence-of-sse-41-and-sse-42-instruction-sets/
// which detects whether `cpuid` is available by checking whether the 21th bit of the EFLAGS register is modifiable or not.
// which detects whether `cpuid` is available by checking whether the 21st bit of the EFLAGS register is modifiable or not.
// If it is, then `cpuid` is available.
let eax: i32;
asm!(r#"
# Save ecx (__fastcall needs it preserved) and save a
# copy of the original eflags that we will restore later:
push %ecx
pushfd
# Copy eflags to ecx and eax:
pushfd
pop %eax
mov %ecx, %eax
# Flip 21st bit and write back to eflags register:
xor %eax, 0x200000
push %eax
popfd
# Read eflags register again:
pushfd
pop %eax
# If cpuid is available, the bit will still be flipped
# and it will be the only bit modified.
#
# xor with the original eflags should produce a 1 in
# the 21st bit in this case, and zeros for all other bits:
xor %eax, %ecx
# So if the value of the 21st bit is 1, cpuid is available,
# and if it is zero, it isn't because we didn't manage to
# modify it:
shrd %eax, 21
# Restore original eflags and ecx:
popfd
pop %ecx
"#
@ -152,10 +166,5 @@ mod tests {
#[test]
fn test_has_cpuid_idempotent() {
assert_eq!(cpuid::has_cpuid(), cpuid::has_cpuid());
let before = __readeflags();
let _ = cpuid::has_cpuid();
let after = __readeflags();
assert_eq!(before, after);
}
}