https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77654
Bug ID: 77654
Summary: restrict pointer attribute not preserved with
-fprefetch-loop-arrays
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: doug.gilmore at imgtec dot com
Target Milestone: ---
Compiling the test example:
void daxpy(int n, double da, double * __restrict dx, double * __restrict dy)
{
int i;
for (i = 0;i < n; i++) {
dy[i] = dy[i] + da*dx[i];
}
}
via:
mips-img-linux-gnu-gcc -fprefetch-loop-arrays daxpy.c -c -O2 -save-temps
-fsched-verbose=9 -fdump-rtl-sched2
The following code is generated for the main loop:
$L4:
ldc1 $f2,0($5)
pref 6,0($2)
ldc1 $f8,-120($2)
addiu $2,$2,32
ldc1 $f6,-144($2)
addiu $5,$5,32
ldc1 $f4,-136($2)
addiu $3,$3,4
maddf.d $f8,$f2,$f0
ldc1 $f2,-128($2)
sdc1 $f8,-152($2)
ldc1 $f1,-24($5)
maddf.d $f6,$f0,$f1
sdc1 $f6,-144($2)
ldc1 $f1,-16($5)
maddf.d $f4,$f0,$f1
sdc1 $f4,-136($2)
ldc1 $f1,-8($5)
maddf.d $f2,$f0,$f1
bne $3,$8,$L4
sdc1 $f2,-128($2)
Due to the __restrict attributes on the pointer declarations, after
scheduling we should see that loads through $5 should move above the
stores through $2. However, during the transformation done by the
phase that is enabled by -fprefetch-loop-arrays, the points-to
information is lost. This prevents the loads to move above the stores
during scheduling.
The attached uses logic borrowed from IVS phase:
0002-Ensure-points-to-information-is-maintained-for-prefe.patch
After applying the patch, the points-to information is maintained,
which results in good code being generated after scheduling (which is
very important when running on in-order processors):
$L4:
addiu $5,$5,32
ldc1 $f8,-120($2)
ldc1 $f6,-112($2)
pref 6,0($2)
ldc1 $f4,-104($2)
addiu $3,$3,4
ldc1 $f2,-96($2)
addiu $2,$2,32
ldc1 $f7,-32($5)
ldc1 $f5,-24($5)
ldc1 $f3,-16($5)
ldc1 $f1,-8($5)
maddf.d $f8,$f7,$f0
maddf.d $f6,$f0,$f5
maddf.d $f4,$f0,$f3
maddf.d $f2,$f0,$f1
sdc1 $f8,-152($2)
sdc1 $f6,-144($2)
sdc1 $f4,-136($2)
sdc1 $f2,-128($2)
bnec $3,$8,$L4
I am not sure what to do about a test case. One possibility is to
commit some of the tracing in debugging patch:
0001-Add-more-tracing-for-missing-points-to-information.patch
and we could scan for the RE "pi. is NULL", in the dump file
created by -fdump-rtl-sched2.
