[Bug translation/64905] New: unsigned short is loaded with 4-byte load (movl)

["r.ayrapetyan at samsung dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64905

            Bug ID: 64905
           Summary: unsigned short is loaded with 4-byte load (movl)
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: translation
          Assignee: unassigned at gcc dot gnu.org
          Reporter: r.ayrapetyan at samsung dot com

Created attachment 34646
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34646&action=edit
Repro case source

Version, target:
  gcc version 5.0.0 20150128 (experimental)
  x86_64-unknown-linux-gnu

Issue:
  In some cases, uint16_t data element is read with 4-byte load (movl
instruction).

Repro case build string:
  gcc -g -Os \
      -ffixed-rax -ffixed-rbx -ffixed-rcx -ffixed-rdx \
      -ffixed-rdi -ffixed-rsi \
      -ffixed-r8 -ffixed-r9 -ffixed-r10 -ffixed-r11 \
      -ffixed-r12 -ffixed-r13 -ffixed-r14 -ffixed-r15 \
      unaligned_read.c -o unaligned_read

Preliminary analysis:
  In the example, ffixed- options are passed to force pointer allocation on the
%rbp register.
  There is another real-world example without ffixed- options, where pointer
was allocated  on the %rbp register and that caused out-of-boundaries memory
access.
  1. The pointer to uint16_t data element was allocated on the %rbp register
     that is marked as aligned to STACK_BOUNDARY.
  2. get_attr_mode called from movhi_internal returns MODE_SI for the
instruction.

This can lead to the following problems:
  1. unaligned memory access (reduced performance);
  2. segmentation fault due to accessing unmapped page (or page mapped with
PROT_NONE)

     // mapped page with array of uint16_t | unmapped page
     function (&array [index_of_last_element_on_the_mapped_page]);
  3. memory access checkers complain about accessing memory out of allocated
array boundaries.