[Bug middle-end/42574] [4.3/4.4/4.5/4.6 Regression] Address of global variable is calculated multiple times (missed CSE)

["mkuvyrkov at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #10 from mkuvyrkov at gcc dot gnu dot org  2010-06-08 15:24 -------
Steven, I'm shamelessly stealing this PR from you.

There are two sides to this missed optimization:

1. Calculation of PIC address is not CSE'd; this is the same as PR42495 and
will be fixed there.
2. Constant "400", which expands to 2 instructions is not CSE'd.

After addressing the first issue, the second problem can be fixed by asking
hoist to CSE "complicated" constants and making sure that RA will rematerialize
them instead of spilling under high register pressure.

We can define a constant "complicated" if it takes a PARALLEL -- (parallel
[(set (reg1) (const)) (clobber reg2)]) -- to set it; this is a common way of
defining instructions that should be split later and require a temporary
register to hold intermediate value.

I'm now testing a patch that makes ARM backend to expand constants into
parallels instead of sequences of two instructions and tweaking hoist to gcse
"complicated" const_int's.

The result is the following:

test:
        push    {r4, r5, r6, lr}
        ldr     r3, .L3
        ldr     r2, .L3+4
.LPIC0:
        add     r3, pc
        ldr     r5, [r3, r2]
        mov     r4, #200
        lsl     r4, r4, #1
        mov     r6, r0
        ldr     r0, [r5, r4]
        bl      func1
        ldr     r0, [r5, r4]
        mov     r1, r6
        bl      func2
        cmp     r0, #0
        beq     .L2
        bl      func
.L2:
        ldr     r0, [r5, r4]
        bl      func3
        @ sp needed for prologue
        pop     {r4, r5, r6, pc}
.L4:
        .align  2
.L3:
        .word   _GLOBAL_OFFSET_TABLE_-(.LPIC0+4)
        .word   glob(GOT)


-- 

mkuvyrkov at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|steven at gcc dot gnu dot   |mkuvyrkov at gcc dot gnu dot
                   |org                         |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42574