https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111875
Bug ID: 111875
Summary: With -Og ubsan check inserted even though
__builtin_assume_aligned guarantees no UB
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: fkastl at suse dot cz
Target Milestone: ---
Host: x86_64-linux
Target: x86_64-linux
Running
gcc -S -Og -fno-sanitize=null -fsanitize=alignment
gcc/testsuite/c-c++-common/ubsan/align-5.c
produces code with an alignment undefined behavior check.
This is how the testcase looks like:
/* { dg-do compile } */
/* { dg-options "-fno-sanitize=null -fsanitize=alignment -O2" } */
/* Check that when optimizing if we know the alignment is right
and we are not doing -fsanitize=null instrumentation we don't
instrument the alignment check. */
__attribute__((noinline, noclone)) int
foo (char *p)
{
p = (char *) __builtin_assume_aligned (p, __alignof__(int));
int *q = (int *) p;
return *q;
}
/* { dg-final { scan-assembler-not "__ubsan_handle" } } */
Because of __builtin_assume_aligned, the compiler should assume that p will
always have the correct alignment to be cast to int *.
The compiler produces this (with -Og):
.file "align-5.c"
.text
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movq %rdi, %rbx
testb $3, %dil
jne .L4
.L2:
movl (%rbx), %eax
popq %rbx
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.L4:
.cfi_restore_state
movq %rdi, %rsi
movl $.Lubsan_data0, %edi
call __ubsan_handle_type_mismatch_v1
jmp .L2
.cfi_endproc
.LFE0:
.size foo, .-foo
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "align-5.c"
.data
.align 32
.type .Lubsan_data0, @object
.size .Lubsan_data0, 32
.Lubsan_data0:
.quad .LC0
.long 12
.long 10
.quad .Lubsan_type0
.byte 2
.byte 0
.zero 6
.align 2
.type .Lubsan_type0, @object
.size .Lubsan_type0, 10
.Lubsan_type0:
.value -1
.value 0
.string "'int'"
.ident "GCC: (GNU) 14.0.0 20231012 (experimental)"
.section .note.GNU-stack,"",@progbits
With -O2 the compiler behaves correctly and produces this:
.file "align-5.c"
.text
.p2align 4
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
movl (%rdi), %eax
ret
.cfi_endproc
.LFE0:
.size foo, .-foo
.ident "GCC: (GNU) 14.0.0 20231012 (experimental)"
.section .note.GNU-stack,"",@progbits
