https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98552
Bug ID: 98552
Summary: Make more use of __builtin_undefined for assuring that
variables do not change
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
Consider
void foo (int *);
void bar (int n)
{
int i;
for (i=0; i<n; i++)
foo(&i);
}
void baz(int n)
{
int i, j;
for (i=0; i<n; i++)
{
j = i;
foo (&i);
if (j /= i)
__builtin_unreachable();
}
}
Assembly for bar and baz are identical, the loop is
.L9:
leaq 12(%rsp), %rdi
call foo
movl 12(%rsp), %eax
addl $1, %eax
movl %eax, 12(%rsp)
cmpl %ebx, %eax
jl .L9
In function bar, things are clear - the value of i has to be
reloaded from the stack, foo might have changed it.
However, this is not possible in baz. j cannot be changed, and the
__builtin_unreachable ensures that i has the same value before and
after the call to foo. It need not be reloaded from the stack.
(The reason why I'm submitting this is another way to approach PR 31593 -
the Fortran front end could annotate code like this to inform
the middle end that DO loops variables are, in fact, invariant).
