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Update documentation

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Jeffrey Walton 2017-11-16 13:36:43 -05:00
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cpu.h
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// cpu.h - originally written and placed in the public domain by Wei Dai
// updated for ARM by Jeffrey Walton
// updated for ARM and PowerPC by Jeffrey Walton.
// updated to split CPU_Query() and CPU_Probe() by Jeffrey Walton.
//! \file cpu.h
//! \brief Functions for CPU features and intrinsics
//! \details The functions are used in X86/X32/X64 and ARM code paths
//! \details The CPU functions are used in IA-32, ARM and PowerPC code paths. The
//! functions provide cpu specific feature testing on IA-32, ARM and PowerPC machines.
//! \details Feature detections uses CPUID on IA-32, like Intel and AMD. On other platforms
//! a two-part strategy is used. First, the library attempts to *Query* the OS for a feature,
//! like using Linux getauxval() or android_getCpuFeatures(). If that fails, then *Probe*
//! the cpu executing an instruction and an observe a SIGILL if unsupported. The general
//! pattern used by the library is:
//! <pre>
//! g_hasCRC32 = CPU_QueryCRC32() || CPU_ProbeCRC32();
//! g_hasPMULL = CPU_QueryPMULL() || CPU_ProbePMULL();
//! g_hasAES = CPU_QueryAES() || CPU_ProbeAES();
//! </pre>
//! \details Generally speaking, CPU_Query() is in the source file <tt>cpu.cpp</tt> because it
//! does not require special architectural flags. CPU_Probe() is in a source file that recieves
//! architectural flags, like <tt>sse-simd.cpp</tt>, <tt>neon-simd.cpp</tt> and
//! <tt>ppc-simd.cpp</tt>. For example, compiling <tt>neon-simd.cpp</tt> an an ARM64 system will
//! have <tt>-march=armv8-a</tt> applied during a compile to make the instruction set architecture
//! (ISA) available.
//! \details The cpu probes are expensive when compared to a standard OS feature query. The library
//! also avoids probes on Apple platforms because Apple's signal handling for SIGILLs appears to
//! corrupt memory. CPU_Probe() will unconditionally return false for Apple platforms. OpenSSL
//! experienced the same problem and moved away from SIGILL probes on Apple.
#ifndef CRYPTOPP_CPU_H
#define CRYPTOPP_CPU_H