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0 pointsrsynnott5y ago0 comments

Micro-architectural differences. For instance, from the 386 to Pentium 3 (and on Athlon, I believe), shifting was cheap, due to the presence of a barrel shifter. This was dropped in the P4, and suddenly shifting was quite expensive (I think, though I’m not sure, that it may actually have come back in later P4s). That’s the sort of thing compiler writers need to know about.

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userbinator5y ago

The P4 was quite an anomaly[1] in the history of x86, optimised for clock speed at the expense of many other things. All the subsequent generations went back to the norm, with the exception of the Atom series, and even that one is not as unusual as the P4.

[1] Even the family/model/stepping designation has an oddity: the 486 was family 4, the Pentium was family 5, and then everything from the Pentium II/III up to the latest Core series, as well as the Atoms, uses family 6; but the P4 was family 15.

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