[Bug math/7006] New: Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[Bug math/7006] New: Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[ilyes dot gouta at gmail dot com]

Hi all,

I hit this bug two years ago while porting a VM on SH-4. The bug is located in
soft-fp/op-2.h, in the macro _FP_FRAC_SRS_2. The bug is in the line 95:

X##_f0 = (X##_f1 << (_FP_W_TYPE_SIZE - (N)) | X##_f0 >> (N) | \

I had to change it to:

X##_f0 = ((X##_f1 << (_FP_W_TYPE_SIZE - (N) - 1)) << 1 | X##_f0 >> (N) | \

Because in the extreme case where you have N == 0 and _FP_W_TYPE_SIZE == 32,
X##_f1 << (_FP_W_TYPE_SIZE - (N)) would return 0, however on SH-4, it returns
X##_f1 since only 5 bits are used in the shift instruction, SHLD, to encode the
shift amount, and 32 (which spans 6 bits) will be masked to 0. This issue can
also be attributed to the SH-4 GCC compiler.

Regards,
Ilyes Gouta.

-- 
           Summary: Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h
                    on SH-4
           Product: glibc
           Version: 2.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: ilyes dot gouta at gmail dot com
                CC: glibc-bugs at sources dot redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=7006

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