The kernel's math-emu code contains a macro _FP_FROM_INT() which is used to convert an integer to a raw normalized floating-point value. It does this basically in three steps: 1. Compute the exponent from the number of leading zero bits. 2. Downshift large fractions to put the MSB in the right position for normalized fractions. 3. Upshift small fractions to put the MSB in the right position. There is an boundary error in step 2, causing a fraction with its MSB exactly one bit above the normalized MSB position to not be downshifted. This results in a non-normalized raw float, which when packed becomes a massively inaccurate representation for that input. The impact of this depends on a number of arch-specific factors, but it is known to have broken emulation of FXTOD instructions on UltraSPARC III, which was originally reported as GCC bug 44631 <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44631>. Any arch which uses math-emu to emulate conversions from integers to same-size floats may be affected. The fix is simple: the exponent comparison used to determine if the fraction should be downshifted must be "<=" not "<". I'm sending a kernel module to test this as a reply to this message. There are also SPARC user-space test cases in the GCC bug entry.
Unified · Split
| @@ -799,7 +799,7 @@ do { \ · 798 unmodified lines | |||
| 799 | X##_e -= (_FP_W_TYPE_SIZE - rsize); \ | 799 | X##_e -= (_FP_W_TYPE_SIZE - rsize); \ |
| 800 | X##_e = rsize - X##_e - 1; \ | 800 | X##_e = rsize - X##_e - 1; \ |
| 801 | \ | 801 | \ |
| 802 | if (_FP_FRACBITS_##fs < rsize && _FP_WFRACBITS_##fs < X##_e) \ | 802 | if (_FP_FRACBITS_##fs < rsize && _FP_WFRACBITS_##fs <= X##_e) \ |
| 803 | __FP_FRAC_SRS_1(ur_, (X##_e - _FP_WFRACBITS_##fs + 1), rsize);\ | 803 | __FP_FRAC_SRS_1(ur_, (X##_e - _FP_WFRACBITS_##fs + 1), rsize);\ |
| 804 | _FP_FRAC_DISASSEMBLE_##wc(X, ur_, rsize); \ | 804 | _FP_FRAC_DISASSEMBLE_##wc(X, ur_, rsize); \ |
| 805 | if ((_FP_WFRACBITS_##fs - X##_e - 1) > 0) \ | 805 | if ((_FP_WFRACBITS_##fs - X##_e - 1) > 0) \ |