diff options
Diffstat (limited to 'absl/base/optimization.h')
| -rw-r--r-- | absl/base/optimization.h | 28 |
1 files changed, 22 insertions, 6 deletions
diff --git a/absl/base/optimization.h b/absl/base/optimization.h index 2fddfc80..0dcbef32 100644 --- a/absl/base/optimization.h +++ b/absl/base/optimization.h @@ -5,7 +5,7 @@ // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // -// http://www.apache.org/licenses/LICENSE-2.0 +// https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, @@ -111,9 +111,9 @@ // See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html // for more information. // -// On some compilers, `ABSL_CACHELINE_ALIGNED` expands to -// `__attribute__((aligned(ABSL_CACHELINE_SIZE)))`. For compilers where this is -// not known to work, the macro expands to nothing. +// On some compilers, `ABSL_CACHELINE_ALIGNED` expands to an `__attribute__` +// or `__declspec` attribute. For compilers where this is not known to work, +// the macro expands to nothing. // // No further guarantees are made here. The result of applying the macro // to variables and types is always implementation-defined. @@ -122,6 +122,14 @@ // of causing bugs that are difficult to diagnose, crash, etc. It does not // of itself guarantee that objects are aligned to a cache line. // +// NOTE: Some compilers are picky about the locations of annotations such as +// this attribute, so prefer to put it at the beginning of your declaration. +// For example, +// +// ABSL_CACHELINE_ALIGNED static Foo* foo = ... +// +// class ABSL_CACHELINE_ALIGNED Bar { ... +// // Recommendations: // // 1) Consult compiler documentation; this comment is not kept in sync as @@ -131,8 +139,10 @@ // 3) Prefer applying this attribute to individual variables. Avoid // applying it to types. This tends to localize the effect. #define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE))) - -#else // not GCC +#elif defined(_MSC_VER) +#define ABSL_CACHELINE_SIZE 64 +#define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE)) +#else #define ABSL_CACHELINE_SIZE 64 #define ABSL_CACHELINE_ALIGNED #endif @@ -153,6 +163,12 @@ // Compilers can use the information that a certain branch is not likely to be // taken (for instance, a CHECK failure) to optimize for the common case in // the absence of better information (ie. compiling gcc with `-fprofile-arcs`). +// +// Recommendation: Modern CPUs dynamically predict branch execution paths, +// typically with accuracy greater than 97%. As a result, annotating every +// branch in a codebase is likely counterproductive; however, annotating +// specific branches that are both hot and consistently mispredicted is likely +// to yield performance improvements. #if ABSL_HAVE_BUILTIN(__builtin_expect) || \ (defined(__GNUC__) && !defined(__clang__)) #define ABSL_PREDICT_FALSE(x) (__builtin_expect(x, 0)) |