[Bug rtl-optimization/46235] inefficient bittest code generation

[tony.poppleton at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46235

--- Comment #4 from Tony Poppleton <tony.poppleton at gmail dot com> 2011-01-28 18:08:15 UTC ---
As a quick test, I commented out the block with the following comment in
fold-const.c:
      /* If this is an EQ or NE comparison with zero and ARG0 is
         (1 << foo) & bar, convert it to (bar >> foo) & 1.  Both require
         two operations, but the latter can be done in one less insn
         on machines that have only two-operand insns or on which a
         constant cannot be the first operand.  */

This produces the following asm code:
        movl    $1, %edx
        movl    %edi, %eax
        movl    %esi, %ecx
        movl    %edx, %edi
        sall    %cl, %edi
        testl   %eax, %edi
        cmove   %edx, %eax
        ret
(using modified GCC 4.6.0 20110122)

So whilst I was hoping for an easy quick-fix, it appears that the required
optimization to convert it into a "btl" test isn't there later on in the
compile.

Incidentally, from looking at http://gmplib.org/~tege/x86-timing.pdf, it
appears that "bt" is slow on P4 architecture (8 cycles if I am reading it
correctly?  which sounds slow), so the llvm code in the bug description isn't
necessarily an optimization on this arch.  Newer chips would probably still
benefit though.