http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46928
Summary: data dependence analysis fails on constant array
accesses
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
In this code we should be able to convert the inner loop to memset zero with
./cc1 -O3 bug.c
typedef int mad_fixed_t;
struct mad_pcm
{
unsigned int samplerate;
unsigned short channels;
unsigned short length;
mad_fixed_t samples[2][1152];
};
struct mad_synth
{
mad_fixed_t filter[2][2][2][16][8];
unsigned int phase;
struct mad_pcm pcm;
};
void mad_synth_mute (struct mad_synth *synth);
void
mad_synth_mute (struct mad_synth *synth)
{
unsigned int ch;
unsigned int s;
unsigned int v;
ch = 0U;
while (ch < 2U)
{
s = 0U;
while (s < 16U)
{
v = 0U;
while (v < 8U)
{
synth->filter[ch][1][1][s][v] = 0;
synth->filter[ch][1][0][s][v] = 0;
synth->filter[ch][0][1][s][v] = 0;
synth->filter[ch][0][0][s][v] = 0;
v++;
}
s++;
}
ch++;
}
return;
}
When looking at the output of the dump
./cc1 -O3 -fdump-tree-ldist-details bug.c
the data dependence analysis fails to analyze the overlapping iterations for
s_29:
(compute_affine_dependence
(stmt_a =
synth_7(D)->filter[ch_28][0][1][s_29][v_30] = 0;
)
(stmt_b =
synth_7(D)->filter[ch_28][0][0][s_29][v_30] = 0;
)
(subscript_dependence_tester
(analyze_overlapping_iterations
(chrec_a = {0, +, 1}_3)
(chrec_b = {0, +, 1}_3)
(overlap_iterations_a = [0]
)
(overlap_iterations_b = [0]
)
)
(analyze_overlapping_iterations
(chrec_a = s_29)
(chrec_b = s_29)
(overlap_iterations_a = not known
)
(overlap_iterations_b = not known
)
)
(dependence classified: scev_not_known)
)
)
When we remove the s loop, we transform the following code with memset zero:
ch = 0U;
while (ch < 2U)
{
v = 0U;
while (v < 8U)
{
synth->filter[ch][1][1][0][v] = 0;
synth->filter[ch][1][0][0][v] = 0;
synth->filter[ch][0][1][0][v] = 0;
synth->filter[ch][0][0][0][v] = 0;
v++;
}
ch++;
}
