https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97225
Bug ID: 97225
Summary: Failure to optimize out conditional addition of zero
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: osandov at osandov dot com
Target Milestone: ---
For the following code:
#include <stddef.h>
struct vector {
int *data;
size_t size;
};
int *vector_end(struct vector *vec)
{
return vec->data + vec->size;
}
GCC 10.2.0 on x86-64 generates the following code (same on -O2, -O3, and -Os):
vector_end:
movq 8(%rdi), %rdx
movq (%rdi), %rax
leaq (%rax,%rdx,4), %rax
ret
However, vector_end() needs to handle empty vectors represented as { NULL, 0 }.
Pointer arithmetic on a null pointer is undefined behavior (even NULL + 0, as
far as I can tell from the C standard), so the correct code is:
int *vector_end(struct vector *vec)
{
if (vec->size == 0)
return vec->data;
return vec->data + vec->size;
}
I'd expect this to generate the same code, but GCC 10.2.0 generates a
conditional move with -O2 and -O3:
vector_end:
movq 8(%rdi), %rdx
movq (%rdi), %rax
testq %rdx, %rdx
leaq (%rax,%rdx,4), %rcx
cmovne %rcx, %rax
ret
And a branch with -Os:
vector_end:
movq 8(%rdi), %rdx
movq (%rdi), %rax
testq %rdx, %rdx
je .L1
leaq (%rax,%rdx,4), %rax
.L1:
ret
Clang 10.0.1, on the other hand, generates the same code with and without the
size check (oddly enough, it also falls back to a conditional move if the size
member is an int or unsigned int instead of size_t/unsigned long):
vector_end: # @vector_end
movq 8(%rdi), %rax
shlq $2, %rax
addq (%rdi), %rax
retq
Can GCC avoid the conditional move/branch here?
