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
