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This is basically what computers do, it's called speculative execution. Modern computers actually have several pipelines, each for a different function. This is

by vault_ 14y ago

This is basically what computers do, it's called speculative execution. Modern computers actually have several pipelines, each for a different function. This is what makes a computer "superscalar". Basically, the CPU tries to keep the pipelines of all of the execution units filled at all times. If it's able to, it will schedule both sides of a branch to be executed, and flush out whichever half wasn't used.

vvhn 14y ago
speculative execution and superscalar are independent concepts. While superscalar does effectively mean multiple pipelines, you don't have to be superscalar for speculative execution. You can reduce pipeline stalls in a single pipeline by "speculating" right as well. The concept that GP is talking was a feature of of Itanium. http://en.wikipedia.org/wiki/Explicitly_parallel_instruction_computing http://en.wikipedia.org/wiki/Explicitly_parallel_instruction... " The EPIC architecture also includes a grab-bag of architectural concepts to increase ILP: Predicated execution is used to decrease the occurrence of branches and to increase the speculative execution of instructions. In this feature, branch conditions are converted to predicate registers which are used to kill results of executed instructions from the side of the branch which is not taken. . . . Multi-way branch instructions improve branch prediction by combining many alternative branches into one bundle. "
wmf 14y ago
Generally speculation only executes one side of the branch. Speculative threading has been proposed using SMT to execute both sides, but usually you're better off trusting the branch predictor and using threads as non-speculative state.
- onan_barbarian 14y ago
  Speculation down both sides is independent of SMT and if I remember correct there were proposals to do this - especially where the branch predictor "knew" it wasn't doing a good job on a branch (no point going down both sides of a 99.9% predicted branch). The problem is that you rapidly run out of execution resources (what if there are more branches to follow, for example) and you will burn lots of energy executing things that don't happen (not to mention speculatively loading things that aren't needed, etc). I remember seeing some papers on the limits of ILP and I think this was one of the 'extreme' ideas (speculation down multiple paths).
  - wmf 14y ago
    To execute both sides, the fetch unit needs to have multiple program counters which is essentially a form of SMT.

by vault_ 14y ago

vvhn 14y ago

speculative execution and superscalar are independent concepts. While superscalar does effectively mean multiple pipelines, you don't have to be superscalar for speculative execution. You can reduce pipeline stalls in a single pipeline by "speculating" right as well. The concept that GP is talking was a feature of of Itanium. http://en.wikipedia.org/wiki/Explicitly_parallel_instruction_computing http://en.wikipedia.org/wiki/Explicitly_parallel_instruction... " The EPIC architecture also includes a grab-bag of architectural concepts to increase ILP: Predicated execution is used to decrease the occurrence of branches and to increase the speculative execution of instructions. In this feature, branch conditions are converted to predicate registers which are used to kill results of executed instructions from the side of the branch which is not taken. . . . Multi-way branch instructions improve branch prediction by combining many alternative branches into one bundle. "

wmf 14y ago

Generally speculation only executes one side of the branch. Speculative threading has been proposed using SMT to execute both sides, but usually you're better off trusting the branch predictor and using threads as non-speculative state.

onan_barbarian 14y ago
Speculation down both sides is independent of SMT and if I remember correct there were proposals to do this - especially where the branch predictor "knew" it wasn't doing a good job on a branch (no point going down both sides of a 99.9% predicted branch). The problem is that you rapidly run out of execution resources (what if there are more branches to follow, for example) and you will burn lots of energy executing things that don't happen (not to mention speculatively loading things that aren't needed, etc). I remember seeing some papers on the limits of ILP and I think this was one of the 'extreme' ideas (speculation down multiple paths).
- wmf 14y ago
  To execute both sides, the fetch unit needs to have multiple program counters which is essentially a form of SMT.