[Bug target/40668] 64-bit sparc miscompiles memcpy of argument inside switch

mikpe at it dot uu dot se gcc-bugzilla@gcc.gnu.org
Tue Jul 7 11:36:00 GMT 2009



------- Comment #3 from mikpe at it dot uu dot se  2009-07-07 11:35 -------
Confirmed, with gcc-4.3-20090705 it works, with gcc-4.4-20090630 it fails.
Compiling with -S and comparing the .s files it looks like 4.4 completely
mis-schedules the code for put_uint32:

put_uint32:
        .register       %g2, #scratch
        .register       %g3, #scratch
        ldub    [%sp+2175], %g1
        ldub    [%sp+2176], %g3
        ldub    [%sp+2177], %g2
        ldub    [%sp+2178], %g4
        st      %o0, [%sp+2175]
        stb     %g4, [%o1+3]
        stb     %g1, [%o1]
        stb     %g3, [%o1+1]
        jmp     %o7+8
         stb    %g2, [%o1+2]

Notice how the store of %o0 to the four bytes at %sp+2175 comes after the
corresponding byte loads, so %g1 to %g4 are loaded with garbage, likely zeroes.

In contrast, gcc-4.3 generates the store before the loads:

put_uint32:
        .register       %g2, #scratch
        .register       %g3, #scratch
        st      %o0, [%sp+2175]
        ldub    [%sp+2176], %g3
        ldub    [%sp+2177], %g4
        ldub    [%sp+2178], %g2
        ldub    [%sp+2175], %g1
        stb     %g2, [%o1+3]
        stb     %g1, [%o1]
        stb     %g3, [%o1+1]
        jmp     %o7+8
         stb    %g4, [%o1+2]


-- 

mikpe at it dot uu dot se changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpe at it dot uu dot se


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40668



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