On 09/30/16 12:14, Bernd Edlinger wrote:
Eric Botcazou wrote:
A comment before the SETs and a testcase would be nice. IIRC
we do have stack size testcases via using -fstack-usage.
Or -Wstack-usage, which might be more appropriate here.
Yes. good idea. I was not aware that we already have that kind of tests.
When trying to write this test. I noticed, that I did not try -Os so far.
But for -Os the stack is still the unchanged 3500 bytes.
However for embedded targets I am often inclined to use -Os, and
would certainly not expect the stack to explode...
I see in arm.md there are places like
/* If we're optimizing for size, we prefer the libgcc calls. */
if (optimize_function_for_size_p (cfun))
FAIL;
Oh, yeah. The comment is completely misleading.
If this pattern fails, expmed.c simply expands some
less efficient rtl, which also results in two shifts
and one or-op. No libgcc calls at all.
So in simple cases without spilling the resulting
assembler is the same, regardless if this pattern
fails or not. But the half-defined out registers
make a big difference when it has to be spilled.
/* Expand operation using core-registers.
'FAIL' would achieve the same thing, but this is a bit smarter. */
scratch1 = gen_reg_rtx (SImode);
scratch2 = gen_reg_rtx (SImode);
arm_emit_coreregs_64bit_shift (LSHIFTRT, operands[0], operands[1],
operands[2], scratch1, scratch2);
.. that explains why this happens. I think it would be better to
use the emit_coreregs for shift count >= 32, because these are
effectively 32-bit shifts.
Will try if that can be improved, and come back with the
results.
The test case with -Os has 3520 bytes stack usage.
When only shift count >= 32 are handled we
have still 3000 bytes stack usage.
And when arm_emit_coreregs_64bit_shift is always
allowed to run, we have 2360 bytes stack usage.
Also for the code size it is better not to fail this
pattern. So I propose to remove this exception in all
three expansions.
Here is an improved patch with the test case from the PR.
And a comment on the redundant SET why it is better to clear
the out register first.
Bootstrap and reg-testing on arm-linux-gnueabihf.
Is it OK for trunk?