Regarding Bug 85539 - x86_64: loads are not always narrowed [https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85539]

[navya deepika Garakapati <navyadeepika.garakapati@gmail.com>]

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.