Match.pd: Make `if (c) a |= CST1 else a &= ~CST1` unconditional.
The compiler transforms conditional bit setting/clearing into an unconditional form, potentially improving performance.
The compiler now transforms code of the form if (c) a |= CST1 else a &= ~CST1 into an unconditional equivalent: (a & ~CST1) | (cond * CST1). This eliminates the conditional branch, which can improve performance. The transformation is only applied when the type size is less than or equal to the machine’s word size to avoid wide integer multiplications.
In Details
This commit adds a new pattern to match.pd that transforms conditional bit manipulation operations into unconditional ones. The transformation replaces a conditional bitwise OR and AND with an equivalent expression that uses bitwise AND, OR, and multiplication. This can improve performance by removing a conditional branch. The condition type <= word_size limits this to avoid unexpected wide integer arithmetic.
For Context
Compilers optimize code by transforming it into equivalent but more efficient forms. Branching (using if statements) can be slow. This commit transforms a conditional bit setting/clearing operation into an unconditional form that avoids the branch. Instead of conditionally setting or clearing a bit, the new code always clears the bit and then sets it again based on the condition. This can execute faster on some processors because it avoids the overhead of conditional execution.