------- Comment #18 from ubizjak at gmail dot com 2007-10-29 13:22 -------
Perhaps this analysis will help someone...:
We start with following loop:
void bar() ()
{
unsigned int ivtmp.19;
int pretmp.14;
struct Foo * pretmp.13;
struct Foo x[4];
int D.1669;
struct Foo * D.1668;
<bb 2>:
<bb 3>:
# ivtmp.19_2 = PHI <ivtmp.19_1(4), 4(2)>
# D.1669_34 = PHI <D.1669_7(4), 3(2)>
# D.1668_33 = PHI <D.1668_6(4), &x[0](2)>
D.1668_33->s = 1;
D.1668_6 = D.1668_33 + 4;
D.1669_7 = D.1669_34 + -1;
ivtmp.19_1 = ivtmp.19_2 - 1;
if (ivtmp.19_1 != 0)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
goto <bb 3>;
...
General ivs in BB are found in tree-ssa-loop-ivopts.c via find_givs_in_bb().
find_givs_in_stmt() is entered with "D.1668_6 = D.1668_33 + 4". For this stmt,
find_givs_in_stmt_scev() returns iv.base = &x[1] and iv.step = 4.
Digging a bit further, find_givs_in_stmt_scev() calls simple_iv() in
tree-scalar-evolution.c() to determine iv base and step. simple_iv() is called
with op = "D.1668_33 + 4" and this op is furhter passed to
analyze_scalar_evolution_in_loop(). This function returns "{&x[1], +, 4}_1".
analyze_scalar_evolution_in_loop() calls analyze_scalar_evolution() with
"D.1668_33 + 4". This expression is further passed to
analyze_scalar_evolution_1(). As the expression is not SSA_NAME, it is passed
to interpret_rhs_modify_stmt() as POINTER_PLUS_EXPR.
interpret_rhs_modify_stmt() decomposes expression into "{&x[0], +, 4}_1" and
"4". These two are summed in chrec_fold_plus() in tree-chrec.c to the final
"{&x[1], +, 4}_1" expression.
This results in:
struct FooD.1649 xD.1666[4];
intD.2 D.1669;
struct FooD.1649 * D.1668;
ivtmp.24D.1709_11 = (unsigned intD.6) &xD.1666[1];
# BLOCK 3 freq:8000
...
MEM[index: ivtmp.24D.1709_9, offset: 0x0fffffffc]{D.1668->sD.1653} = 1;
ivtmp.24D.1709_10 = ivtmp.24D.1709_9 + 4;
...
and on i686:
leal -12(%ebp), %eax
leal 4(%ebp), %edx
.L2:
movl $1, -4(%eax)
addl $4, %eax
cmpl %edx, %eax
jne .L2
...
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26726
