On Sat, Sep 27, 2014 at 8:09 AM, Jordan Justen <jljus...@gmail.com> wrote: > On Fri, Sep 26, 2014 at 6:09 PM, Matt Turner <matts...@gmail.com> wrote: >> ... which leads to incorrect results on 32-bit x86. >> >> Reported-by: Mark Janes <mark.a.ja...@intel.com> >> --- >> I tried writing up a nice commit message that explained what was going >> on and why this worked on 64-bit, but then I realized that it was taking >> orders of magnitude longer than the fix itself and probably no one would >> care anyway. > > No cliff notes version? :)
Type promotion rules promote the int in 'int / sizeof(...)' to unsigned, since sizeof(...) is unsigned. Since the particular sizeofs in question are powers of two, the compiler optimizes this into right shifts. Unfortunately, since the possibly negative numerator has now been promoted to unsigned, the compiler will choose a logical shift right instruction rather than arithmetic. On x86-64 gcc generates this code (before): sar $0x20,%rax ; int jump = brw_inst_imm_d(brw, insn); mov %rax,%rsi shr $0x3,%rsi ; int jump_compacted = jump / sizeof(brw_compact_inst); shr $0x4,%rax ; int jump_uncompacted = jump / sizeof(brw_inst); (after): shr $0x20,%rax ; ; int jump = brw_inst_imm_d(brw, insn); ; some exception case handling, and %eax copied to %ecx sar $0x3,%ecx ; int jump_compacted = jump / sizeof(brw_compact_inst); sar $0x4,%eax ; int jump_uncompacted = jump / sizeof(brw_inst); On 64-bit platforms, there's no functional difference in this case, since the high 32-bits of the 64-bit register are zero, but on 32-bit platforms (this is how far I got, but this part seems unclear to me) Basically, the unsigned promotion causes gcc to choose shr rather than sar, and for some reasons that I haven't quite wrapped my head around this doesn't cause problems on x86-64, but does on x86-32. > Is it something that you could give a few lines of code that reproduce it? I tried, and couldn't. :( > Reviewed-by: Jordan Justen <jordan.l.jus...@intel.com> I've sent two patches that replace this one that hopefully make the code a lot clearer. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
