Fwd: Using movw/movt rather than minipools in ARM gcc

[David Sehr <sehr at google dot com>]

Hello All,

We are using gcc trunk as of 4/27/12, and are attempting to add
support to the ARM gcc compiler for Native Client.
We are trying to get gcc -march=armv7-a to use?movw/movt consistently
instead of minipools.?The motivation is for
a new target variant where armv7-a is the minimum?supported and
non-code in .text is never allowed (per Native Client rules).
But the?current behavior looks like a generically poor optimization
for -march=armv7-a. ?(Surely memory loads are slower
than movw/movt, and no space is saved in many cases.) ?For further
details, this seems to only happen with -O2 or higher.
-O1 generates movw/movt, seemingly because cprop is folding away a
LO_SUM/HIGH pair. ?Another data point to note
is that?"Ubuntu/Linaro 4.5.2-8ubuntu3" does produce?movw/movt for this
test case, but we haven't tried stock 4.5.

I have enabled TARGET_USE_MOVT, which should force?a large fraction of
constant materialization to use movw/movt
rather than pc-relative loads. ?However, I am still seeing pc-relative
loads for the?following example case and am looking
for help from the experts here.

int a[1000], b[1000], c[1000];

void foo(int n) {
? int i;
? for (i = 0; i < n; ++i) {
? ? a[i] = b[i] + c[i];
? }
}

When I compile this I get:

foo:
? ? ? ? ...
ldr r3, .L7
ldr r1, .L7+4
ldr r2, .L7+8
? ? ? ? ...
.L7:
.word ?b
.word ?c
.word ?a
.size foo, .-foo
.comm c,4000,4
.comm b,4000,4
.comm a,4000,4

From some investigation, it seems I need to add a define_split to
convert SYMBOL_REFs to LO_SUM/HIGH pairs.
There is already a function called arm_split_constant that seems to do
this, but no rule seems to be firing to cause
it to get invoked. ?Before I dive into writing the define_split, am I
missing something obvious?

Cheers,

David