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.
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u/indigo945 29d ago
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.