Date: Wed, 15 Aug 2012 19:34:23 +0000 Subject: Vendor import of llvm trunk r161861: http://llvm.org/svn/llvm-project/llvm/trunk@161861 --- docs/BranchWeightMetadata.html | 164 ----------------------------------------- 1 file changed, 164 deletions(-) delete mode 100644 docs/BranchWeightMetadata.html (limited to 'docs/BranchWeightMetadata.html') diff --git a/docs/BranchWeightMetadata.html b/docs/BranchWeightMetadata.html deleted file mode 100644 index 38b87ba..0000000 --- a/docs/BranchWeightMetadata.html +++ /dev/null @@ -1,164 +0,0 @@ - - - - - LLVM Branch Weight Metadata - - - - -

- LLVM Branch Weight Metadata -

- -

Introduction
Supported Instructions
Built-in "expect" Instruction
CFG Modifications

- -

Written by Jakub Staszak

- -

- Introduction -

Branch Weight Metadata represents branch weights as its likeliness to -be taken. Metadata is assigned to the TerminatorInst as a -MDNode of the MD_prof kind. The first operator is always a -MDString node with the string "branch_weights". Number of operators -depends on the terminator type.

- -

Branch weights might be fetch from the profiling file, or generated based on -__builtin_expect instruction. -

- -

All weights are represented as an unsigned 32-bit values, where higher value -indicates greater chance to be taken.

- -

- Supported Instructions -

- -

BranchInst

Metadata is only assign to the conditional branches. There are two extra - operarands, for the true and the false branch.

-!0 = metadata !{
-  metadata !"branch_weights",
-  i32 <TRUE_BRANCH_WEIGHT>,
-  i32 <FALSE_BRANCH_WEIGHT>
-}
-

- -

SwitchInst

Branch weights are assign to every case (including default case - which is always case #0).

-!0 = metadata !{
-  metadata !"branch_weights",
-  i32 <DEFAULT_BRANCH_WEIGHT>
-  [ , i32 <CASE_BRANCH_WEIGHT> ... ]
-}
-

- -

IndirectBrInst

Branch weights are assign to every destination.

-!0 = metadata !{
-  metadata !"branch_weights",
-  i32 <LABEL_BRANCH_WEIGHT>
-  [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
-}
-

- -

Other

Other terminator instructions are not allowed to contain Branch Weight - Metadata.

- -

- Built-in "expect" Instructions -

__builtin_expect(long exp, long c) instruction provides branch - prediction information. The return value is the value of exp.

- -

It is especially useful in conditional statements. Currently Clang supports - two conditional statements: -

`if` statement

The exp parameter is the condition. The c parameter is - the expected comparision value. If it is equal to 1 (true), the condition is - likely to be true, in other case condition is likely to be false. For example: -

-  if (__builtin_expect(x > 0, 1)) {
-    // This block is likely to be taken.
-  }
-

- -

`switch` statement

The exp parameter is the value. The c parameter is the - expected value. If the expected value doesn't show on the cases list, the - default case is assumed to be likely taken.

-  switch (__builtin_expect(x, 5)) {
-  default: break;
-  case 0:  // ...
-  case 3:  // ...
-  case 5:  // This case is likely to be taken.
-  }
-

- -

- CFG Modifications -

Branch Weight Metatada is not proof against CFG changes. If terminator -operands' are changed some action should be taken. In other case some -misoptimizations may occur due to incorrent branch prediction information.

- -

- - Jakub Staszak
- LLVM Compiler Infrastructure
-

- - - -- cgit v1.1