[llvm-bugs] [Bug 45707] New: unaligned Memory Access Identify

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45707

            Bug ID: 45707
           Summary: unaligned Memory Access Identify
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: 2077213809 at qq.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

When I write code to access a specified address that is not 4-byte aligned,The
compiler still thinks he's 4-byte aligned.

code:
void test() {
  int *p = (int *)0x101;
  *p = 2;
}

cmd: clang -O2 --target=arm-none-eabi -mcpu=cortex-r52 -emit-llvm -S test.c
test.ll
llvm IR: -- the store is align 4！
define dso_local void @test() local_unnamed_addr #0 {
  store i32 2, i32* inttoptr (i32 257 to i32*), align 4, !tbaa !3
  ret void
}

Even if I don't allow unaligned access, the compiler won't split into ldrb.
CMD: llc -mattr=+strict-align test.ll
ASM: 
test:
  .fnstart
@ %bb.0:
  mov r0, #1
  mov r1, #2
  orr r0, r0, #256
  str r1, [r0]
  bx  lr

GCC can identify the misaligned access.
CMD: arm-none-eabi-gcc -mcpu=cortex-r52 -munaligned-access -O2 -S test.c
test:
  @ args = 0, pretend = 0, frame = 0
  @ frame_needed = 0, uses_anonymous_args = 0
  @ link register save eliminated.
  mov r3, #0
  mov r2, #2
  str r2, [r3, #257]  @ unaligned
  bx  lr

CMD:  arm-none-eabi-gcc -mcpu=cortex-r52 -mno-unaligned-access -O2 -S test.c
test:
  @ args = 0, pretend = 0, frame = 0
  @ frame_needed = 0, uses_anonymous_args = 0
  @ link register save eliminated.
  mov r3, #0
  mov r2, #2
  strb  r3, [r3, #258]
  strb  r3, [r3, #259]
  strb  r3, [r3, #260]
  strb  r2, [r3, #257]
  bx  lr

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