On 12/23/14 19:47, H.J. Lu wrote:
On Tue, Dec 23, 2014 at 6:16 PM, John David Anglin <dave.ang...@bell.net> wrote:
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.
I run gcc.dg/pr55023.c on x86-64 and x86 without your fix. It works
fine. Can you find a testcase, failing on x86-64 and x86, which is
fixed by your change? If not, this bug doesn't affect x86 and you
should find another way to fix your target problem without introducing
a regression on x86.
Just to clarify, this is not a target problem, this is a problem that
DSE is not designed to handle certain situations WRT argument stores
that occur on targets with certain properties.
Jeff
