Optimize bitwise operations for signed integers to -1
GCC now simplifies `(~a) >> a` to `-1` for signed integer `a`, reducing unnecessary computations during compilation.
This commit introduces an optimization in match.pd where the expression (~a) >> a simplifies to -1 when a is a signed integer. This transformation is based on the mathematical equivalence ~(a>>a) for right shifts of complement operations. By recognizing this pattern and directly substituting -1, the compiler eliminates redundant bitwise operations, resulting in more efficient code generation.
In Details
The match.pd file defines tree-level optimizations in GCC. This specific optimization rewrites (~a) >> a to -1 for signed a. This relies on the property that ~a (all bits flipped) then right-shifted by a (which effectively sign-extends) will result in an all-ones bit pattern, which is -1 in two's complement. This fold simplifies the intermediate representation, allowing for more streamlined code generation by reducing complex bitwise operations to a constant.
For Context
Compilers often look for mathematical shortcuts to make your programs run faster. This change in GCC (within the match.pd file) introduces one such shortcut for signed integers. If your code calculates (~a) >> a – which means 'bitwise NOT 'a', then right-shift by 'a'' – the compiler now recognizes that for signed numbers, this sequence always results in the value -1. By knowing this, the compiler can directly insert -1 instead of performing the actual bitwise operations, leading to slightly faster and more compact code.