Hi All,
For the below testcase,
$cat test.c
int foo(long *p)
{
return *p;
}
when compile with clang -O2
$clang test.c -O2 -S
$cat test.s
foo:
movl (%rdi), %eax
retq
while gcc gives
$gcc test.c -O2 -S
$cat test.s
foo:
movq (%rdi), %rax
retq
As you can see the difference clang is loading value in eax(32 bit) while
gcc uses rax(64 bit).
While analysing it we found that ,
In gcc trunck with -O0 optimization,
we can notice that in ira pass, there is a conversion information from
64bit to 32bit (subreg:SI (reg:DI 82 [ _1 ]) 0)):
ira pass:
(insn 8 7 11 2 (set (reg:SI 83 [ _4 ])
(subreg:SI (reg:DI 82 [ _1 ]) 0)) "t.c":3:8 67 {*movsi_internal}
(expr_list:REG_DEAD (reg:DI 82 [ _1 ])
(nil)))
while reload pass is deleting this information and assigning 32bit to 32bit
i.e; SI to SI which results this as a dead code for the next pass and as a
result this insn 8 is getting deleted in next pass which is split2 pass.
reload pass:
(insn 8 7 15 2 (set (reg:SI 0 ax [orig:83 _4 ] [83])
(reg:SI 0 ax [orig:82 _1 ] [82])) "t.c":3:8 67 {*movsi_internal}
(nil))
We would like to know why conversion from DI to SI is converted to SI to
SI in reload pass (which uses any target hook).
Thanks & Regards
Navya.
