[Bug middle-end/18424] [3.4/4.0 Regression] ~6x+ performance regression, constant trees not being computed.

["roger at eyesopen dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From roger at eyesopen dot com  2004-12-09 14:59 -------
The patch is a "partial" fix as there will still be a performance regression for
the code generated vs. gcc 3.3.1.  The reason being that 3.3.1 generated
incorrect code for test program in this PR.

int foo(int a) { return (a & (1L<<23)) ? 1 : 2; }

is supposed to sign-extend "a" from a 16-bit integer to a 32-bit long, to match
the (long) constant operand.  This sign-extension may end setting bit 23, so the
result of the function is dependent upon "a".  i.e. the best we can do is:

int foo(int a) { return (a < 0) ? 1 : 2; }

Unfortunately, we don't quite get that efficient for avr-elf, instead still
producing the sign-extension and an AND by constant.  This fixes the regression
aspects of this patch, but we still have a missed optimization.  The code we
generate is:

        clr r26
        sbrc r25,7
        com r26
        mov r27,r26
        sbrs r26,7
        rjmp .L2
        ldi r24,lo8(1)
        ldi r25,hi8(1)
        ret
.L2:
        ldi r24,lo8(2)
        ldi r25,hi8(2)
        ret

Perhaps once the patch is committed, we should close this PR, and open a
separate "enhancement" request PR to catch this missed AND(SIGN_EXTEND ..))
opportunity on AVR.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18424