[Bug target/85539] New: x86_64: loads are not always narrowed

[tilkax at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85539

            Bug ID: 85539
           Summary: x86_64: loads are not always narrowed
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tilkax at gmail dot com
  Target Milestone: ---

When casting a 64-bit memory value to 32 bits, it's possible to only load the
lower 32 bits (this also avoids the REX prefix). GCC doesn't always do this:
https://godbolt.org/g/EzqKNo


#include <cstdint>

uint32_t foo(uint64_t *p)
{
    return *p;
}


Actual:
        mov     rax, QWORD PTR [rdi]
        ret

Expected:
        mov     eax, DWORD PTR [rdi]
        ret