Implement fninit - #2778
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fninit (DB E3) is missing from the x87 decode table, so it raises SIGILL, while its immediate neighbour fnclex (DB E2) is there. Reported in ish-app#2415, where it kills the Free Pascal compiler at startup: fpc emits fninit in its FPU setup, so nothing compiled with it runs. It resets the control word to 0x037f and clears the status word. Clearing fsw also resets TOP, since TOP is a field of it, which is what empties the register stack; there is no tag word to write because none is modelled. The control word changing means the live rounding mode has to follow it, the same way fldcw does. Verified against real Linux with the same static i386 binary, after dirtying the FPU with a non-default control word and a non-empty stack: before after / real Linux fninit SIGILL runs control word -- 0x037f status word (incl TOP) -- 0x0000 FPU usable afterwards -- yes (The test reads the status word with fnstsw ax rather than fnstsw m16, because the memory form is also unimplemented -- a separate gap.)
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fninit(DB E3) is missing from the x87 decode table, so it raises SIGILL, while its immediate neighbourfnclex(DB E2) is there.Reported in #2415, where it kills the Free Pascal compiler at startup: fpc emits
fninitin its FPU setup, so nothing compiled with it runs.What it does
Resets the control word to
0x037fand clears the status word. Clearingfswalso resets TOP, since TOP is a field of it, which is what empties the register stack; there is no tag word to write because none is modelled. The control word changing means the live rounding mode has to follow it, the same wayfldcwdoes.Verification
Same static i386 binary run on real Linux and on iSH, after dirtying the FPU with a non-default control word and a non-empty register stack:
fninit0x037f0x0000Reproduced on the Alpine 3.19.0 rootfs the App Store build downloads (
ROOTFS_URLinapp/iSH.xcconfig), against master 7864dd6.Note that
fpcis not packaged for x86 in 3.19, so this is the instruction on its own rather than that program; #2415 is the report, the table above is the check.The test reads the status word with
fnstsw axrather thanfnstsw m16, because the memory form is also unimplemented. That is a separate gap.