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* [Bug target/45026] New: Empty function compiles to many loads and stores
@ 2010-07-21 21:06 steven at gcc dot gnu dot org
2010-07-26 20:08 ` [Bug target/45026] " sje at cup dot hp dot com
0 siblings, 1 reply; 4+ messages in thread
From: steven at gcc dot gnu dot org @ 2010-07-21 21:06 UTC (permalink / raw)
To: gcc-bugs
Consider this test case:
typedef struct teststruct
{
double d;
char f1;
int arr[15];
} teststruct;
void
copystruct5 (teststruct param)
{
return;
}
The mighty copystruct5 function does absolutely nothing, so I expect this to be
compiled to absolutely nothing. But not so with r162374 at -O2:
.file "t.c"
.pred.safe_across_calls p1-p5,p16-p63
.text
.align 16
.align 64
.global copystruct5#
.type copystruct5#, @function
.proc copystruct5#
copystruct5:
.prologue
.spill 48
.mmi
.fframe 48
adds r12 = -48, r12
[.L2:]
.body
;;
mov r14 = r12
adds r15 = 16, r12
;;
.mmi
st8 [r14] = r32, 8
st8 [r15] = r34
nop 0
;;
.mmi
st8 [r14] = r33
nop 0
adds r14 = 24, r12
;;
.mmi
st8 [r14] = r35
nop 0
adds r14 = 32, r12
;;
.mmi
st8 [r14] = r36
nop 0
adds r14 = 40, r12
;;
.mmi
st8 [r14] = r37
nop 0
adds r14 = 48, r12
;;
.mmi
st8 [r14] = r38
nop 0
adds r14 = 56, r12
;;
.mib
st8 [r14] = r39
.restore sp
adds r12 = 48, r12
br.ret.sptk.many b0
.endp copystruct5#
.ident "GCC: (GNU) 4.6.0 20100721 (experimental) [trunk revision
162374]"
What's that?!
--
Summary: Empty function compiles to many loads and stores
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: steven at gcc dot gnu dot org
GCC target triplet: ia64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45026
^ permalink raw reply [flat|nested] 4+ messages in thread* [Bug target/45026] Empty function compiles to many loads and stores
2010-07-21 21:06 [Bug target/45026] New: Empty function compiles to many loads and stores steven at gcc dot gnu dot org
@ 2010-07-26 20:08 ` sje at cup dot hp dot com
0 siblings, 0 replies; 4+ messages in thread
From: sje at cup dot hp dot com @ 2010-07-26 20:08 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from sje at cup dot hp dot com 2010-07-26 20:07 -------
It doesn't result in any loads, just stores. This code is storing the first
part of the structure argument which was passed (partially) in registers into
memory. Obviously it doesn't need to do this since the argument isn't used. I
think this is an unfortunate side-effect of passing a large structure as an
argument. If the structure were smaller and fit entirely into registers you
would see an empty function. Large structures are split on IA64 with the first
part going into registers and the last part going into memory and it looks like
GCC 'reconstructs' the structure in memory before determining if it is needed
or not.
Ideally, you should pass a pointer here instead of passing (and copying) the
structure, but it would be nice if GCC realised the argument wasn't needed and
didn't do the stores so this could be considered a missed optimization.
--
sje at cup dot hp dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |sje at cup dot hp dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45026
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-08-14 21:31 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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[not found] <bug-45026-4@http.gcc.gnu.org/bugzilla/>
2011-02-25 23:17 ` [Bug target/45026] Empty function compiles to many loads and stores steven at gcc dot gnu.org
2021-08-14 21:30 ` [Bug rtl-optimization/45026] struct passed as argument in memory compiles to dead stores pinskia at gcc dot gnu.org
2021-08-14 21:31 ` pinskia at gcc dot gnu.org
2010-07-21 21:06 [Bug target/45026] New: Empty function compiles to many loads and stores steven at gcc dot gnu dot org
2010-07-26 20:08 ` [Bug target/45026] " sje at cup dot hp dot com
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