//! x86 run-time feature detection is OS independent.

#[cfg(target_arch = "x86")]
use core::arch::x86::*;
#[cfg(target_arch = "x86_64")]
use core::arch::x86_64::*;

use core::mem;

use crate::detect::{bit, cache, Feature};

/// Run-time feature detection on x86 works by using the CPUID instruction.
///
/// The [CPUID Wikipedia page][wiki_cpuid] contains
/// all the information about which flags to set to query which values, and in
/// which registers these are reported.
///
/// The definitive references are:
/// - [Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2:
///   Instruction Set Reference, A-Z][intel64_ref].
/// - [AMD64 Architecture Programmer's Manual, Volume 3: General-Purpose and
///   System Instructions][amd64_ref].
///
/// [wiki_cpuid]: https://en.wikipedia.org/wiki/CPUID
/// [intel64_ref]: http://www.intel.de/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
/// [amd64_ref]: http://support.amd.com/TechDocs/24594.pdf
#[allow(clippy::similar_names)]
pub(crate) fn detect_features() -> cache::Initializer {
    let mut value = cache::Initializer::default();

    // If the x86 CPU does not support the CPUID instruction then it is too
    // old to support any of the currently-detectable features.
    if !has_cpuid() {
        return value;
    }

    // Calling `__cpuid`/`__cpuid_count` from here on is safe because the CPU
    // has `cpuid` support.

    // 0. EAX = 0: Basic Information:
    // - EAX returns the "Highest Function Parameter", that is, the maximum
    // leaf value for subsequent calls of `cpuinfo` in range [0,
    // 0x8000_0000]. - The vendor ID is stored in 12 u8 ascii chars,
    // returned in EBX, EDX, and   ECX (in that order):
    let (max_basic_leaf, vendor_id) = unsafe {
        let CpuidResult {
            eax: max_basic_leaf,
            ebx,
            ecx,
            edx,
        } = __cpuid(0);
        let vendor_id: [[u8; 4]; 3] = [
            mem::transmute(ebx),
            mem::transmute(edx),
            mem::transmute(ecx),
        ];
        let vendor_id: [u8; 12] = mem::transmute(vendor_id);
        (max_basic_leaf, vendor_id)
    };

    if max_basic_leaf < 1 {
        // Earlier Intel 486, CPUID not implemented
        return value;
    }

    // EAX = 1, ECX = 0: Queries "Processor Info and Feature Bits";
    // Contains information about most x86 features.
    let CpuidResult {
        ecx: proc_info_ecx,
        edx: proc_info_edx,
        ..
    } = unsafe { __cpuid(0x0000_0001_u32) };

    // EAX = 7, ECX = 0: Queries "Extended Features";
    // Contains information about bmi,bmi2, and avx2 support.
    let (extended_features_ebx, extended_features_ecx) = if max_basic_leaf >= 7 {
        let CpuidResult { ebx, ecx, .. } = unsafe { __cpuid(0x0000_0007_u32) };
        (ebx, ecx)
    } else {
        (0, 0) // CPUID does not support "Extended Features"
    };

    // EAX = 0x8000_0000, ECX = 0: Get Highest Extended Function Supported
    // - EAX returns the max leaf value for extended information, that is,
    // `cpuid` calls in range [0x8000_0000; u32::MAX]:
    let CpuidResult {
        eax: extended_max_basic_leaf,
        ..
    } = unsafe { __cpuid(0x8000_0000_u32) };

    // EAX = 0x8000_0001, ECX=0: Queries "Extended Processor Info and Feature
    // Bits"
    let extended_proc_info_ecx = if extended_max_basic_leaf >= 1 {
        let CpuidResult { ecx, .. } = unsafe { __cpuid(0x8000_0001_u32) };
        ecx
    } else {
        0
    };

    {
        // borrows value till the end of this scope:
        let mut enable = |r, rb, f| {
            if bit::test(r as usize, rb) {
                value.set(f as u32);
            }
        };

        enable(proc_info_ecx, 0, Feature::sse3);
        enable(proc_info_ecx, 1, Feature::pclmulqdq);
        enable(proc_info_ecx, 9, Feature::ssse3);
        enable(proc_info_ecx, 13, Feature::cmpxchg16b);
        enable(proc_info_ecx, 19, Feature::sse4_1);
        enable(proc_info_ecx, 20, Feature::sse4_2);
        enable(proc_info_ecx, 23, Feature::popcnt);
        enable(proc_info_ecx, 25, Feature::aes);
        enable(proc_info_ecx, 29, Feature::f16c);
        enable(proc_info_ecx, 30, Feature::rdrand);
        enable(extended_features_ebx, 18, Feature::rdseed);
        enable(extended_features_ebx, 19, Feature::adx);
        enable(extended_features_ebx, 11, Feature::rtm);
        enable(proc_info_edx, 4, Feature::tsc);
        enable(proc_info_edx, 23, Feature::mmx);
        enable(proc_info_edx, 24, Feature::fxsr);
        enable(proc_info_edx, 25, Feature::sse);
        enable(proc_info_edx, 26, Feature::sse2);
        enable(extended_features_ebx, 29, Feature::sha);

        enable(extended_features_ebx, 3, Feature::bmi1);
        enable(extended_features_ebx, 8, Feature::bmi2);

        // `XSAVE` and `AVX` support:
        let cpu_xsave = bit::test(proc_info_ecx as usize, 26);
        if cpu_xsave {
            // 0. Here the CPU supports `XSAVE`.

            // 1. Detect `OSXSAVE`, that is, whether the OS is AVX enabled and
            // supports saving the state of the AVX/AVX2 vector registers on
            // context-switches, see:
            //
            // - [intel: is avx enabled?][is_avx_enabled],
            // - [mozilla: sse.cpp][mozilla_sse_cpp].
            //
            // [is_avx_enabled]: https://software.intel.com/en-us/blogs/2011/04/14/is-avx-enabled
            // [mozilla_sse_cpp]: https://hg.mozilla.org/mozilla-central/file/64bab5cbb9b6/mozglue/build/SSE.cpp#l190
            let cpu_osxsave = bit::test(proc_info_ecx as usize, 27);

            if cpu_osxsave {
                // 2. The OS must have signaled the CPU that it supports saving and
                // restoring the:
                //
                // * SSE -> `XCR0.SSE[1]`
                // * AVX -> `XCR0.AVX[2]`
                // * AVX-512 -> `XCR0.AVX-512[7:5]`.
                //
                // by setting the corresponding bits of `XCR0` to `1`.
                //
                // This is safe because the CPU supports `xsave`
                // and the OS has set `osxsave`.
                let xcr0 = unsafe { _xgetbv(0) };
                // Test `XCR0.SSE[1]` and `XCR0.AVX[2]` with the mask `0b110 == 6`:
                let os_avx_support = xcr0 & 6 == 6;
                // Test `XCR0.AVX-512[7:5]` with the mask `0b1110_0000 == 224`:
                let os_avx512_support = xcr0 & 224 == 224;

                // Only if the OS and the CPU support saving/restoring the AVX
                // registers we enable `xsave` support:
                if os_avx_support {
                    // See "13.3 ENABLING THE XSAVE FEATURE SET AND XSAVE-ENABLED
                    // FEATURES" in the "Intel® 64 and IA-32 Architectures Software
                    // Developer’s Manual, Volume 1: Basic Architecture":
                    //
                    // "Software enables the XSAVE feature set by setting
                    // CR4.OSXSAVE[bit 18] to 1 (e.g., with the MOV to CR4
                    // instruction). If this bit is 0, execution of any of XGETBV,
                    // XRSTOR, XRSTORS, XSAVE, XSAVEC, XSAVEOPT, XSAVES, and XSETBV
                    // causes an invalid-opcode exception (#UD)"
                    //
                    enable(proc_info_ecx, 26, Feature::xsave);

                    // For `xsaveopt`, `xsavec`, and `xsaves` we need to query:
                    // Processor Extended State Enumeration Sub-leaf (EAX = 0DH,
                    // ECX = 1):
                    if max_basic_leaf >= 0xd {
                        let CpuidResult {
                            eax: proc_extended_state1_eax,
                            ..
                        } = unsafe { __cpuid_count(0xd_u32, 1) };
                        enable(proc_extended_state1_eax, 0, Feature::xsaveopt);
                        enable(proc_extended_state1_eax, 1, Feature::xsavec);
                        enable(proc_extended_state1_eax, 3, Feature::xsaves);
                    }

                    // FMA (uses 256-bit wide registers):
                    enable(proc_info_ecx, 12, Feature::fma);

                    // And AVX/AVX2:
                    enable(proc_info_ecx, 28, Feature::avx);
                    enable(extended_features_ebx, 5, Feature::avx2);

                    // For AVX-512 the OS also needs to support saving/restoring
                    // the extended state, only then we enable AVX-512 support:
                    if os_avx512_support {
                        enable(extended_features_ebx, 16, Feature::avx512f);
                        enable(extended_features_ebx, 17, Feature::avx512dq);
                        enable(extended_features_ebx, 21, Feature::avx512ifma);
                        enable(extended_features_ebx, 26, Feature::avx512pf);
                        enable(extended_features_ebx, 27, Feature::avx512er);
                        enable(extended_features_ebx, 28, Feature::avx512cd);
                        enable(extended_features_ebx, 30, Feature::avx512bw);
                        enable(extended_features_ebx, 31, Feature::avx512vl);
                        enable(extended_features_ecx, 1, Feature::avx512vbmi);
                        enable(extended_features_ecx, 5, Feature::avx512bf16);
                        enable(extended_features_ecx, 6, Feature::avx512vbmi2);
                        enable(extended_features_ecx, 8, Feature::avx512gfni);
                        enable(extended_features_ecx, 8, Feature::avx512vp2intersect);
                        enable(extended_features_ecx, 9, Feature::avx512vaes);
                        enable(extended_features_ecx, 10, Feature::avx512vpclmulqdq);
                        enable(extended_features_ecx, 11, Feature::avx512vnni);
                        enable(extended_features_ecx, 12, Feature::avx512bitalg);
                        enable(extended_features_ecx, 14, Feature::avx512vpopcntdq);
                    }
                }
            }
        }

        // This detects ABM on AMD CPUs and LZCNT on Intel CPUs.
        // On intel CPUs with popcnt, lzcnt implements the
        // "missing part" of ABM, so we map both to the same
        // internal feature.
        //
        // The `is_x86_feature_detected!("lzcnt")` macro then
        // internally maps to Feature::abm.
        enable(extended_proc_info_ecx, 5, Feature::lzcnt);

        // As Hygon Dhyana originates from AMD technology and shares most of the architecture with
        // AMD's family 17h, but with different CPU Vendor ID("HygonGenuine")/Family series
        // number(Family 18h).
        //
        // For CPUID feature bits, Hygon Dhyana(family 18h) share the same definition with AMD
        // family 17h.
        //
        // Related AMD CPUID specification is https://www.amd.com/system/files/TechDocs/25481.pdf.
        // Related Hygon kernel patch can be found on
        // http://lkml.kernel.org/r/5ce86123a7b9dad925ac583d88d2f921040e859b.1538583282.git.puwen@hygon.cn
        if vendor_id == *b"AuthenticAMD" || vendor_id == *b"HygonGenuine" {
            // These features are available on AMD arch CPUs:
            enable(extended_proc_info_ecx, 6, Feature::sse4a);
            enable(extended_proc_info_ecx, 21, Feature::tbm);
        }
    }

    // Unfortunately, some Skylake chips erroneously report support for BMI1 and
    // BMI2 without actual support. These chips don't support AVX, and it seems
    // that all Intel chips with non-erroneous support BMI do (I didn't check
    // other vendors), so we can disable these flags for chips that don't also
    // report support for AVX.
    //
    // It's possible this will pessimize future chips that do support BMI and
    // not AVX, but this seems minor compared to a hard crash you get when
    // executing an unsupported instruction (to put it another way, it's safe
    // for us to under-report CPU features, but not to over-report them). Still,
    // to limit any impact this may have in the future, we only do this for
    // Intel chips, as it's a bug only present in their chips.
    //
    // This bug is documented as `SKL052` in the errata section of this document:
    // http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/desktop-6th-gen-core-family-spec-update.pdf
    if vendor_id == *b"GenuineIntel" && !value.test(Feature::avx as u32) {
        value.unset(Feature::bmi1 as u32);
        value.unset(Feature::bmi2 as u32);
    }

    value
}