On 23-Dec-14, at 7:28 PM, H.J. Lu wrote:
On Tue, Dec 23, 2014 at 3:55 PM, John David Anglin <dave.ang...@bell.net
> wrote:
On 23-Dec-14, at 5:37 PM, H.J. Lu wrote:
In this case, arguments are passed in registers. Isn't the
optimization
disabled for ia32, which passes arguments on stack, even before your
change?
It's not disabled in dse.c. Possibly, this occurs for some cases
for ia32
in ix86_function_ok_for_sibcall.
The problem in dse is in how to distinguish stores used for
arguments from
those used for general
manipulation of data. It seems the argument data for the call to
bar in the
testcase are copied through
frame memory on x86_64.
We have two situations:
/* This field is only used for the processing of const functions.
These functions cannot read memory, but they can read the stack
because that is where they may get their parms. We need to be
this conservative because, like the store motion pass, we don't
consider CALL_INSN_FUNCTION_USAGE when processing call insns.
Moreover, we need to distinguish two cases:
1. Before reload (register elimination), the stores related to
outgoing arguments are stack pointer based and thus deemed
of non-constant base in this pass. This requires special
handling but also means that the frame pointer based stores
need not be killed upon encountering a const function call.
2. After reload, the stores related to outgoing arguments can be
either stack pointer or hard frame pointer based. This means
that we have no other choice than also killing all the frame
pointer based stores upon encountering a const function call.
This field is set after reload for const function calls. Having
this set is less severe than a wild read, it just means that all
the frame related stores are killed rather than all the
stores. */
bool frame_read;
Case 1 is incorrect for sibling calls as the call arguments are
frame or
argument pointer based
when they are not passed in registers.
When we don't have a const function, dse assumes:
/* Every other call, including pure functions, may read any
memory
that is not relative to the frame. */
add_non_frame_wild_read (bb_info);
Again, this is incorrect for sibling calls (i.e., dse in general
assumes
that call arguments are register
or stack pointer based before reload).
This optimization is done on purpose to fix:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44194
Unless it is also incorrect for x86, why not only disable it for
your target without penalizing x86 through a target hook.
I tend to think the old code was wrong for x86 but I recognize the
lost optimization.
This isn't an optimization issue on hppa. It's a wrong code bug.
Richard wrote regarding 44194:
"So we create a stack representation to copy it to the stack temporary
(which both
are unneeded). We should see that we can avoid the temporary at all as
there is no
aggregate use of the struct left. At least we should avoid the 2nd
temporary."
The temporaries are still there and they are the problem.
I don't see that this is a target issue. There were nothing more than
argument stores in
the testcase for the PR 55023 and they were deleted by dse.
Dave
--
John David Anglin dave.ang...@bell.net
