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.
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>
}
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> ... ]
}
Branch weights are assign to every destination.
!0 = metadata !{
metadata !"branch_weights",
i32 <LABEL_BRANCH_WEIGHT>
[ , i32 <LABEL_BRANCH_WEIGHT> ... ]
}
Other terminator instructions are not allowed to contain Branch Weight Metadata.
__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:
The exp
parameter is the condition. The c
parameter is the expected
comparison 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.
}
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.
}
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.