On Wed, 22 Apr 2020, Richard Biener wrote:
>
> This extends DECL_GIMPLE_REG_P to all types so we can clear
> TREE_ADDRESSABLE even for integers with partial defs, not just
> complex and vector variables. To make that transition easier
> the patch inverts DECL_GIMPLE_REG_P to DECL_NOT_GIMPLE_REG_P
> since that makes the default the current state for all other
> types besides complex and vectors. That also nicely simplifies
> code throughout the compiler.
>
> TREE_ADDRESSABLE and DECL_NOT_GIMPLE_REG_P are now truly
> independent, either set prevents a decl from being rewritten
> into SSA form.
>
> For the testcase in PR94703 we're able to expand the partial
> def'ed local integer to a register then, producing a single
> movl rather than going through the stack.
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> If there are no objections I'm going to install this once
> stage1 opens.
Of course there was some fallout. On 32bit x86 gcc.dg/torture/pr71522.c
fails execution because while the GIMPLE is unchanged at RTL expansion
time:
main ()
{
char s[12];
long double d;
MEM <unsigned char[12]> [(char * {ref-all})&d] = MEM <unsigned char[12]>
[(char * {ref-all})"AAAAAAAAAAA"];
MEM <unsigned char[12]> [(char * {ref-all})&s] = MEM <unsigned char[12]>
[(char * {ref-all})&d];
_1 = __builtin_strcmp (&s, "AAAAAAAAAAA");
if (_1 != 0)
...
we now assign 'd' an XFmode register (TREE_ADDRESSABLE is cleared
now since we can set DECL_NOT_GIMPLE_REG_P). The case is lost
then, impossible to fix up AFAICS. On x86 all moves to/from
XFmode are normalizing, specifically we end up with
fldt .LC0
fstpt (%esp)
now the most appealing solution - and totally in the opposite
direction of this patch - is to simply stop expanding non-SSA names
as pseudos. I do not remember the history as why we do this
but it's likely remanents we preserved from either pre-SSA, times
we did not go into SSA for -O0 or times we really gone out-of-SSA.
There is _some_ good reason to expand a non-SSA "register" into
a pseudo though - namely that RTL is not SSA and thus can accept
partial defs. And of course that RTL cannot get rid of a stack
slot assigned to a variable. Today we have somewhat robust
infrastructure to deal with partial defs on GIMPLE, namely
BIT_INSERT_EXPR, but it's not fully exercised.
It's of course possible to fixup the above problematical
cases (there's precenent with discover_nonconstant_array_refs,
which could be "easily" extended to handle "weird" accesses
of non-integral-mode variables) but with the recent discussion
on making RTL expansion more straight-forward I'd bring up
the above idea ... it would get rid of quite some special
code dealing with tcc_reference trees (and MEM_REFs) ending
up operating on registers.
Thanks,
Richard.
