base: disable SIMD on MSVC x86_64 by default

This recreates the case as of commit f169822 (tag v0.4.0-alpha.4), in
that, by default (without #define'ing a macro or passing an /arch:ETC
compiler flag), Wuffs does not use SIMD on MSVC x86_64.

Commit b64a761 (after tag v0.4.0-alpha.4, before tag v0.4.0-alpha.5)
changed the default so that x86_64_v2 (roughly equivalent to SSE4.2) was
enabled by default, since the user from issue #148 was enabling that
anyway (in an unsupported way, by #define'ing a macro that was a private
implementation detail) with no problems (and better performance).

However, another user later reported (in issue #151) that enabling SIMD
on MSVC x86_64 somehow lead to ICEs (Internal Compiler Errors).

This commit restores the default to "no SIMD" and it is up to the MSVC
user to opt in to the SIMD code paths. Clang and GCC are unaffected:
SIMD remains enabled by default.

Updates #148
Updates #151

Author: nigeltao

doc/changelog.md

@@ -26,7 +26,10 @@ The LICENSE has changed from a single license (Apache 2) to a dual license
 - Added `std/xxhash32`.
 - Added `std/xxhash64`.
 - Added `std/xz`.
+- Added `WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY`.
 - Added `WUFFS_CONFIG__DST_PIXEL_FORMAT__ENABLE_ALLOWLIST`.
+- Added `WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V2`.
+- Added `WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V3`.
 - Added `wuffs_base__status__is_truncated_input_error`.
 - Changed `lzw.set_literal_width` to `lzw.set_quirk`.
 - Changed `set_quirk_enabled!(quirk: u32, enabled: bool)` to `set_quirk!(key:

internal/cgen/base/fundamental-public.h

@@ -119,12 +119,62 @@
 #elif defined(_MSC_VER)  // (#if-chain ref AVOID_CPU_ARCH_1)
 
 #if defined(_M_X64)
-// We need <intrin.h> for the __cpuid function.
-#include <intrin.h>
+
+// On X86_64, Microsoft Visual C/C++ (MSVC) only supports SSE2 by default.
+// There are /arch:SSE2, /arch:AVX and /arch:AVX2 compiler flags (the AVX2 one
+// is roughly equivalent to X86_64_V3), but there is no /arch:SSE42 compiler
+// flag that's equivalent to X86_64_V2.
+//
+// For getting maximum performance with X86_64 MSVC and Wuffs, pass /arch:AVX2
+// (and then test on the oldest hardware you intend to support).
+//
+// Absent that compiler flag, either define one of the three macros listed
+// below or else the X86_64 SIMD code will be disabled and you'll get a #pragma
+// message stating this library "performs best with /arch:AVX2". This message
+// is harmless and ignorable, in that the non-SIMD code is still correct and
+// reasonably performant, but is a reminder that when combining Wuffs and MSVC,
+// some compiler configuration is required for maximum performance.
+//
+//  - WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY
+//  - WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V2 (enables SSE4.2 and below)
+//  - WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V3 (enables AVX2 and below)
+//
+// Defining the first one (WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+// or defining none of those three (the default state) are equivalent (in that
+// both disable the SIMD code paths), other than that pragma message.
+//
+// When defining these WUFFS_CONFIG__ENABLE_ETC macros with MSVC, be aware that
+// some users report it leading to ICEs (Internal Compiler Errors), but other
+// users report no problems at all (and improved performance). It's unclear
+// exactly what combination of SIMD code and MSVC configuration lead to ICEs.
+// Do your own testing with your own MSVC version and configuration.
+//
+// https://github.com/google/wuffs/issues/148
+// https://github.com/google/wuffs/issues/151
+// https://developercommunity.visualstudio.com/t/fatal--error-C1001:-Internal-compiler-er/10703305
+//
+// Clang (including clang-cl) and GCC don't need this WUFFS_CONFIG__ETC macro
+// machinery, or having the Wuffs-the-library user to fiddle with compiler
+// flags, because they support "__attribute__((target(arg)))".
+#if defined(__AVX2__) || defined(__clang__) || \
+    defined(WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V3)
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V2
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V3
+#elif defined(WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V2)
 #define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64
 #define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V2
-#if defined(__AVX2__) || defined(__clang__)
+#elif !defined(WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+#pragma message("Wuffs with MSVC+X64 performs best with /arch:AVX2")
+#endif  // defined(__AVX2__) || defined(__clang__) || etc
 
+#if defined(WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64)
+
+#if defined(WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+#error "MSVC_CPU_ARCH simultaneously enabled and disabled"
+#endif
+
+#include <intrin.h>
 // intrin.h isn't enough for X64 SIMD, with clang-cl, if we want to use
 // "__attribute__((target(arg)))" without e.g. "/arch:AVX".
 //
@@ -134,23 +184,9 @@
 #include <immintrin.h>  // AVX, AVX2, FMA, POPCNT
 #include <nmmintrin.h>  // SSE4.2
 #include <wmmintrin.h>  // AES, PCLMUL
-#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V3
 
-#else  // defined(__AVX2__) || defined(__clang__)
-
-// clang-cl (which defines both __clang__ and _MSC_VER) supports
-// "__attribute__((target(arg)))".
-//
-// For MSVC's cl.exe (unlike clang or gcc), SIMD capability is a compile-time
-// property of the source file (e.g. a /arch:AVX2 or -mavx2 compiler flag), not
-// of individual functions (that can be conditionally selected at runtime).
-#if !defined(WUFFS_CONFIG__I_KNOW_THAT_WUFFS_MSVC_PERFORMS_BEST_WITH_ARCH_AVX2)
-#pragma message("Wuffs with MSVC+IX86/X64 performs best with /arch:AVX2")
-#endif
-
-#endif  // defined(__AVX2__) || defined(__clang__)
+#endif  // defined(WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64)
 #endif  // defined(_M_X64)
-
 #endif  // (#if-chain ref AVOID_CPU_ARCH_1)
 #endif  // (#if-chain ref AVOID_CPU_ARCH_0)
 

release/c/wuffs-unsupported-snapshot.c

@@ -175,12 +175,62 @@ extern "C" {
 #elif defined(_MSC_VER)  // (#if-chain ref AVOID_CPU_ARCH_1)
 
 #if defined(_M_X64)
-// We need <intrin.h> for the __cpuid function.
-#include <intrin.h>
+
+// On X86_64, Microsoft Visual C/C++ (MSVC) only supports SSE2 by default.
+// There are /arch:SSE2, /arch:AVX and /arch:AVX2 compiler flags (the AVX2 one
+// is roughly equivalent to X86_64_V3), but there is no /arch:SSE42 compiler
+// flag that's equivalent to X86_64_V2.
+//
+// For getting maximum performance with X86_64 MSVC and Wuffs, pass /arch:AVX2
+// (and then test on the oldest hardware you intend to support).
+//
+// Absent that compiler flag, either define one of the three macros listed
+// below or else the X86_64 SIMD code will be disabled and you'll get a #pragma
+// message stating this library "performs best with /arch:AVX2". This message
+// is harmless and ignorable, in that the non-SIMD code is still correct and
+// reasonably performant, but is a reminder that when combining Wuffs and MSVC,
+// some compiler configuration is required for maximum performance.
+//
+//  - WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY
+//  - WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V2 (enables SSE4.2 and below)
+//  - WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V3 (enables AVX2 and below)
+//
+// Defining the first one (WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+// or defining none of those three (the default state) are equivalent (in that
+// both disable the SIMD code paths), other than that pragma message.
+//
+// When defining these WUFFS_CONFIG__ENABLE_ETC macros with MSVC, be aware that
+// some users report it leading to ICEs (Internal Compiler Errors), but other
+// users report no problems at all (and improved performance). It's unclear
+// exactly what combination of SIMD code and MSVC configuration lead to ICEs.
+// Do your own testing with your own MSVC version and configuration.
+//
+// https://github.com/google/wuffs/issues/148
+// https://github.com/google/wuffs/issues/151
+// https://developercommunity.visualstudio.com/t/fatal--error-C1001:-Internal-compiler-er/10703305
+//
+// Clang (including clang-cl) and GCC don't need this WUFFS_CONFIG__ETC macro
+// machinery, or having the Wuffs-the-library user to fiddle with compiler
+// flags, because they support "__attribute__((target(arg)))".
+#if defined(__AVX2__) || defined(__clang__) || \
+    defined(WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V3)
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V2
+#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V3
+#elif defined(WUFFS_CONFIG__ENABLE_MSVC_CPU_ARCH__X86_64_V2)
 #define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64
 #define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V2
-#if defined(__AVX2__) || defined(__clang__)
+#elif !defined(WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+#pragma message("Wuffs with MSVC+X64 performs best with /arch:AVX2")
+#endif  // defined(__AVX2__) || defined(__clang__) || etc
 
+#if defined(WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64)
+
+#if defined(WUFFS_CONFIG__DISABLE_MSVC_CPU_ARCH__X86_64_FAMILY)
+#error "MSVC_CPU_ARCH simultaneously enabled and disabled"
+#endif
+
+#include <intrin.h>
 // intrin.h isn't enough for X64 SIMD, with clang-cl, if we want to use
 // "__attribute__((target(arg)))" without e.g. "/arch:AVX".
 //
@@ -190,23 +240,9 @@ extern "C" {
 #include <immintrin.h>  // AVX, AVX2, FMA, POPCNT
 #include <nmmintrin.h>  // SSE4.2
 #include <wmmintrin.h>  // AES, PCLMUL
-#define WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64_V3
 
-#else  // defined(__AVX2__) || defined(__clang__)
-
-// clang-cl (which defines both __clang__ and _MSC_VER) supports
-// "__attribute__((target(arg)))".
-//
-// For MSVC's cl.exe (unlike clang or gcc), SIMD capability is a compile-time
-// property of the source file (e.g. a /arch:AVX2 or -mavx2 compiler flag), not
-// of individual functions (that can be conditionally selected at runtime).
-#if !defined(WUFFS_CONFIG__I_KNOW_THAT_WUFFS_MSVC_PERFORMS_BEST_WITH_ARCH_AVX2)
-#pragma message("Wuffs with MSVC+IX86/X64 performs best with /arch:AVX2")
-#endif
-
-#endif  // defined(__AVX2__) || defined(__clang__)
+#endif  // defined(WUFFS_PRIVATE_IMPL__CPU_ARCH__X86_64)
 #endif  // defined(_M_X64)
-
 #endif  // (#if-chain ref AVOID_CPU_ARCH_1)
 #endif  // (#if-chain ref AVOID_CPU_ARCH_0)