CODEWITHSUNDEEP
macosclang
wjk
wjk

Reputation: 1249

Clang 5.1 fpmath

I am trying to compile a program with Clang 5.1, as included in Xcode 5.1. This program is an early-stage boot loader, and as such its execution environment is very limited. I must pass the -mfpmath=387 compiler flag to produce correct assembly. When I upgraded to Xcode 5.1, I received the following error:

error: the '387' unit is not supported with this instruction set

Does anyone know what this error means? Did the syntax for this flag change, and, if so, what is the new syntax? (I am also interested in knowing what -mfpmath=387 does. I copied it verbatim from a Makefile in boot-132, but never really understood its effect on the compilation procedure.)

Upvotes: 6

Views: 1471

Answers (2)

Chris Devereux
Chris Devereux

Reputation: 5473

Educated guess:

The 387 refers to the intel x87 floating point instruction set http://en.wikipedia.org/wiki/X87 (hence the 'fpmath' bit of the compiler flag).

It's likely that the flag is telling the compiler to generate code targeting the 387 floating point architecture, rather than a later version.

Upvotes: 1

wjk
wjk

Reputation: 1249

As it turns out, for Clang to accept -mfpmath=387, I had to also pass -mno-sse. I found this out by grepping Clang’s source. I still want to know what -mfpmath=387 does, though.

Upvotes: 6

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