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).