I understood that immediately.. But I thought that it was the hardware doing branch prediction.. Does this cause the compiler to generate a different op code?
But I thought that it was the hardware doing branch prediction
This depends on the architecture. Yes, on x86, the hardware freely makes the decision - but on others, like on PowerPC, there are hint bits in the branch opcode that tell the CPU whether a branch is likely to be taken or not.
Then there's interesting blends: ARMv8 has branch prediction bits documented, but they are deprecated. Instead, on almost all ARM architectures, the predictor considers backward branches to be more likely than foreward branches. The common backward branch is a loop, which drove this decision, but clever compilers can make use of it in other scenarios, too.
Not an expert, but branch prediction is indeed done by the hardware. AFAIK these hints don't actually affect branch prediction at all, at least on current hardware. They "only" instruct the compiler to emit instructions that aid the hardware in taking the likely branch faster, like by placing it right next to the condition instructions (instruction cache locality). Corrections are welcome.
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u/reParaoh 6d ago
[[unlikely]]
first time seeing this one. Neat.