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DIVS.L of $80000000 by -1 in the 32-bit form used a native i32
division, which panics ("attempt to divide with overflow") and takes
the emulator down. A program feeding edge cases to the Toolbox
fixed-point routines hit this on a Mac II with System 4.1.
While there:
- the 64-bit signed form zero-extended the divisor, so a negative
divisor gave a wrong quotient;
- overflow was only tested on the 64-bit forms, and accepted any
quotient whose high long was 0 or $FFFFFFFF: an unsigned quotient
above 32 bits, or a signed one outside the i32 range, went through.
Do signed divisions in 64 bits with a sign-extended divisor, checked
for i64::MIN / -1, and report overflow (V set, operands unchanged)
whenever the quotient doesn't fit in 32 bits.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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DIVS.L of $80000000 by -1 in the 32-bit form used a native i32 division causes a panics: "attempt to divide with overflow".
It takes the emulator down. A program feeding edge cases to the Toolbox fixed-point routines hit this on a Mac II with System 4.1 (among others).
While there:
Do signed divisions in 64 bits with a sign-extended divisor, checked for i64::MIN / -1, and report overflow (V set, operands unchanged) whenever the quotient doesn't fit in 32 bits.
Note: This was mostly found by AI after debugging a crash and the fix written by AI with a bit of guidance and review from me.