https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70359
Aldy Hernandez <aldyh at gcc dot gnu.org> changed:
What |Removed |Added
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Assignee|unassigned at gcc dot gnu.org |aldyh at gcc dot gnu.org
--- Comment #30 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
Created attachment 43597
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43597&action=edit
untested patch implementing suggestion in comment 26
The attached untested patch attempts to implement the suggestion in comment 26
of replacing the out-of-loop pre-inc with post-inc values.
Richi, is this more or less what you had in mind?
Assuming this:
LOOP:
# p_8 = PHI <p_16(2), p_19(3)>
...
p_19 = p_8 + 4294967295;
goto LOOP:
The patch replaces:
p_22 = p_8 + 4294967294;
MEM[(char *)p_19 + 4294967295B] = 45;
into:
p_22 = p_19 + 4294967295;
*p_22 = 45;
This allows the backend to use auto-dec in two places:
strb r1, [r4, #-1]!
...
strblt r3, [r4, #-1]!
...reducing the byte count from 116 to 104, but just shy of the 96 needed to
eliminate the regression. I will discuss the missing bytes in a follow-up
comment, as they are unrelated to this IV adjustment patch.
It is worth noting that x86 also benefits from a reduction of 3 bytes with this
patch, as we remove 2 lea instructions: one within the loop, and one before
returning. Thus, I believe this is a regression across the board, or at least
in multiple architectures.
A few comments...
While I see the benefit of hijacking insert_backedge_copies() for this, I am
not a big fan of changing the IL after the last tree dump (*t.optimized), as
the modified IL would only be visible in *r.expand. Could we perhaps move this
to another spot? Say after the last forwprop pass, or perhaps right before
expand? Or perhaps have a *t.final dump right before expand?
As mentioned, this is only a proof of concept. I made the test rather
restrictive. I suppose we could relax the conditions and generalize it a bit.
There are comments throughout showing what I had in mind.
