Hi Richi,
Following up your suggestion on PR41488, I am continuing to test with
loop header copy before FRE in GCC 4.4.1. One regression I am trying
to tackle is from the test case below.
typedef struct {
int degree;
int c[(256)];
} swbcht_Polynomial;
typedef struct {
int m;
int n;
const swbcht_Polynomial *primPoly;
unsigned short alpha[(1 << (13))];
} swbcht_GF;
typedef struct {
swbcht_GF gf;
} swbcht_Handle;
static const swbcht_Polynomial _primPoly13;
void _ComputeGFLookupTables (swbcht_Handle *bchH, int m)
{
int i, j;
swbcht_GF *gf = &bchH->gf;
gf->m = m;
gf->n = (1 << m) - 1;
gf->primPoly = &_primPoly13;
for (i = 0; i < gf->m; i++) {
gf->alpha[gf->m] ^= (gf->primPoly->c[i] * gf->alpha[i]);
}
}
The dump after CH - FRE looks like
_ComputeGFLookupTables (struct swbcht_Handle * bchH, int m)
{
int i;
short unsigned int D.1241;
short int D.1240;
short int D.1239;
short unsigned int D.1238;
short unsigned int D.1237;
short unsigned int D.1236;
const int D.1235;
const struct swbcht_Polynomial * D.1233;
short int D.1232;
short unsigned int D.1231;
int D.1230;
int D.1229;
int D.1228;
<bb 2>:
bchH_2(D)->gf.m = m_4(D);
D.1228_5 = 1 << m_4(D);
D.1229_6 = D.1228_5 + -1;
bchH_2(D)->gf.n = D.1229_6;
bchH_2(D)->gf.primPoly = &_primPoly13;
if (m_4(D) > 0)
goto <bb 5>;
else
goto <bb 6>;
<bb 6>:
goto <bb 4>;
<bb 5>:
<bb 3>:
# i_35 = PHI <0(5), i_23(7)>
D.1230_10 = bchH_2(D)->gf.m;
D.1231_11 = bchH_2(D)->gf.alpha[D.1230_10];
D.1232_12 = (short int) D.1231_11;
D.1233_13 = bchH_2(D)->gf.primPoly;
D.1235_15 = D.1233_13->c[i_35];
D.1236_16 = (short unsigned int) D.1235_15;
D.1237_18 = bchH_2(D)->gf.alpha[i_35];
D.1238_19 = D.1236_16 * D.1237_18;
D.1239_20 = (short int) D.1238_19;
D.1240_21 = D.1239_20 ^ D.1232_12;
D.1241_22 = (short unsigned int) D.1240_21;
bchH_2(D)->gf.alpha[D.1230_10] = D.1241_22;
i_23 = i_35 + 1;
D.1230_8 = bchH_2(D)->gf.m;
if (i_23 < D.1230_8)
goto <bb 7>;
else
goto <bb 8>;
<bb 7>:
goto <bb 3>;
<bb 8>:
<bb 4>:
return;
}
1. Why does FRE miss eliminating expression bchH_2(D)->gf.primPoly in
bb 3 with &_primPoly13. This is normally the case if we ran FRE then
CH.
Further PRE identifies bchH_2(D)->gf.primPoly as a partially redundant
expression and hoists it into predecessor bb 7 and we get
<bb 3>:
# i_35 = PHI <0(5), i_23(7)>
# prephitmp.25_49 = PHI <m_4(D)(5), D.1230_8(7)>
# prephitmp.27_51 = PHI <&_primPoly13(5), pretmp.26_50(7)>
D.1230_10 = prephitmp.25_49;
D.1231_11 = bchH_2(D)->gf.alpha[D.1230_10];
D.1232_12 = (short int) D.1231_11;
D.1233_13 = prephitmp.27_51;
D.1235_15 = D.1233_13->c[i_35];
D.1236_16 = (short unsigned int) D.1235_15;
D.1237_18 = bchH_2(D)->gf.alpha[i_35];
D.1238_19 = D.1236_16 * D.1237_18;
D.1239_20 = (short int) D.1238_19;
D.1240_21 = D.1239_20 ^ D.1232_12;
D.1241_22 = (short unsigned int) D.1240_21;
bchH_2(D)->gf.alpha[D.1230_10] = D.1241_22;
i_23 = i_35 + 1;
D.1230_8 = bchH_2(D)->gf.m;
if (D.1230_8 > i_23)
goto <bb 7>;
else
goto <bb 8>;
<bb 7>:
pretmp.26_50 = bchH_2(D)->gf.primPoly;
goto <bb 3>;
Clearly prephitmp.27_51 will make a redundant induction variable.
Stepping through the insert_into_preds_of_block in tree-ssa-pre.c
I can see the value numbers for expression bchH_2(D)->gf.primPoly
available through bb 3 and through bb 2 are different.
2. Is this because alias analysis cannot determine bchH_2(D)->gf
has a unique target?
Many Thanks,
Rahul
