[Bug target/50794] New: [picochip] incorrect implementation of ashrsi3 for negative numbers

[Paulo.Matos at csr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50794

             Bug #: 50794
           Summary: [picochip] incorrect implementation of ashrsi3 for
                    negative numbers
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: paulo.ma...@csr.com


I was looking at the ashrsi3 implementation in picochip and I find something
odd. I think it would generate incorrect results for negative numbers. Note,
however that I have not tested the code, so this is through inspection only.

Picochip ashrsi3.c contains the following piece of code:

      /* Given a word of sign bits, shift back left to create the
         destination sign bits. */
      wordOfSignBits = __builtin_asri(value.s.high, 15);
      signWord = wordOfSignBits << reverseCount;
      result.s.high |= signWord;

Note however that wordOfSignBits should be 0xffff for negative value.s.high and
0x0000 for positive value.s.high. However, this will either be 0x0000 for
positive value.s.high or 0x0001 for negative value.s.high. This is because
value.s.high is unsigned, so arithmetic shift will be performed as if on an
unsigned value and the word will never contain the expected value.

Replacing the above by:
      /* Given a word of sign bits, shift back left to create the
         destination sign bits. */
+      HItype topword = value.s.high;
+      wordOfSignBits = __builtin_asri(topword, 15);
-      wordOfSignBits = __builtin_asri(value.s.high, 15);
       signWord = wordOfSignBits << reverseCount;
       result.s.high |= signWord;