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Expression costs are required to be given in terms of COSTS_N_INSNS (n),
which is defined to stand for the count of single fast instructions, and
actually returns `n * 4'. The VAX backend however instead operates on
naked numbers, causing an anomaly for the integer const zero rtx, where
the cost given is 4 as opposed to 1 for integers in the [1:63] range, as
well as -1 for comparisons. This is because the value of 0 returned by
`vax_rtx_costs' is converted to COSTS_N_INSNS (1) in `pattern_cost':
return cost > 0 ? cost : COSTS_N_INSNS (1);
Consequently, where feasible, 1 or -1 are preferred over 0 by the middle
end causing code pessimization, e.g. rather than producing this:
subl2 $4,%sp
movl 4(%ap),%r0
jgtr .L2
addl2 $2,%r0
.L2:
ret
or this:
subl2 $4,%sp
addl3 4(%ap),8(%ap),%r0
jlss .L6
addl2 $2,%r0
.L6:
ret
code is produced like this:
subl2 $4,%sp
movl 4(%ap),%r0
cmpl %r0,$1
jgeq .L2
addl2 $2,%r0
.L2:
ret
or this:
subl2 $4,%sp
addl3 4(%ap),8(%ap),%r0
cmpl %r0,$-1
jleq .L6
addl2 $2,%r0
.L6:
ret
from this:
int
compare_mov (int x)
{
if (x > 0)
return x;
else
return x + 2;
}
and this:
int
compare_add (int x, int y)
{
int z;
z = x + y;
if (z < 0)
return z;
else
return z + 2;
}
respectively, which is slower and larger both at a time.
Furthermore once the backend is converted to MODE_CC this anomaly makes
it usually impossible to remove redundant comparisons in the comparison
elimination pass, because most VAX instructions set the condition codes
as per the relation of the instruction's result to 0 and not -1.
The middle end has some other assumptions as to rtx costs being given in
terms of COSTS_N_INSNS, so wrap all the VAX rtx costs then as they stand
into COSTS_N_INSNS invocations, effectively scaling the costs by 4 while
preserving their relative values, except for the integer const zero rtx
given the value of `COSTS_N_INSNS (1) / 2', half of a fast instruction
(this can be further halved if needed in the future).
Adjust address costs likewise so that they remain proportional to the
new absolute values of rtx costs.
Code size stats are as follows, collected from 17639 executables built
in `check-c' GCC testing:
samples average median
--------------------------------------
regressions 1420 0.400% 0.195%
unchanged 13811 0.000% 0.000%
progressions 2408 -0.504% -0.201%
--------------------------------------
total 17639 -0.037% 0.000%
with a small number of outliers only (over 5% size change):
old new change %change filename
----------------------------------------------------
4991 5249 258 5.1693 981001-1.exe
2637 2777 140 5.3090 interchange-6.exe
2187 2307 120 5.4869 sprintf.x7
3969 4197 228 5.7445 pr28982a.exe
8264 8816 552 6.6795 vector-compare-1.exe
5199 5575 376 7.2321 pr28982b.exe
2113 2411 298 14.1031 20030323-1.exe
2113 2411 298 14.1031 20030323-1.exe
2113 2411 298 14.1031 20030323-1.exe
so it seems we are looking good, and we have complementing reductions
to compensate:
old new change %change filename
----------------------------------------------------
2919 2631 -288 -9.8663 pr57521.exe
3427 3167 -260 -7.5868 sabd_1.exe
2985 2765 -220 -7.3701 ssad-run.exe
2985 2765 -220 -7.3701 ssad-run.exe
2985 2765 -220 -7.3701 usad-run.exe
2985 2765 -220 -7.3701 usad-run.exe
4509 4253 -256 -5.6775 vshuf-v2sf.exe
4541 4285 -256 -5.6375 vshuf-v2si.exe
4673 4417 -256 -5.4782 vshuf-v2df.exe
2993 2841 -152 -5.0785 abs-2.x4
2993 2841 -152 -5.0785 abs-3.x4
This actually causes `loop-8.c' to regress:
FAIL: gcc.dg/loop-8.c scan-rtl-dump-times loop2_invariant "Decided" 1
FAIL: gcc.dg/loop-8.c scan-rtl-dump-not loop2_invariant "without introducing a new temporary register"
but upon a closer inspection this is a red herring. Old code looks as
follows:
.file "loop-8.c"
.text
.align 1
.globl f
.type f, @function
f:
.word 0
subl2 $4,%sp
movl 4(%ap),%r2
movl 8(%ap),%r3
movl $42,(%r2)
clrl %r0
movl $42,%r1
movl %r1,%r4
jbr .L2
.L5:
movl %r4,%r1
.L2:
movl %r1,(%r3)[%r0]
incl %r0
cmpl %r0,$100
jeql .L6
movl $42,(%r2)[%r0]
bicl3 $-2,%r0,%r1
jeql .L5
movl %r0,%r1
jbr .L2
.L6:
ret
.size f, .-f
while new one is like below:
.file "loop-8.c"
.text
.align 1
.globl f
.type f, @function
f:
.word 0
subl2 $4,%sp
movl 4(%ap),%r2
movl $42,(%r2)+
movl 8(%ap),%r1
clrl %r0
movl $42,%r3
movzbl $100,%r4
movl %r3,%r5
jbr .L2
.L5:
movl %r5,%r3
.L2:
movl %r3,(%r1)+
incl %r0
cmpl %r0,%r4
jeql .L6
movl $42,(%r2)+
bicl3 $-2,%r0,%r3
jeql .L5
movl %r0,%r3
jbr .L2
.L6:
ret
.size f, .-f
and is clearly better: not only it is smaller, but it also uses the
post-increment rather than indexed addressing mode in the loop, of
which the former comes for free in terms of both performance and code
size while the latter causes an extra byte per operand to be produced
for the index register and also incurs an execution penalty for the
extra address calculation.
Exclude the case from VAX testing then, as already done for some other
targets and discussed with commit d242fdaec186 ("gcc.dg/loop-8.c: Skip
for mmix.").
gcc/
* config/vax/vax.c (vax_address_cost): Express the cost in terms
of COSTS_N_INSNS.
(vax_rtx_costs): Likewise.
gcc/testsuite/
* gcc.dg/loop-8.c: Exclude for `vax-*-*'.
* gcc.target/vax/compare-add-zero.c: New test.
* gcc.target/vax/compare-mov-zero.c: New test.
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