Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches

Matthew Wilcox
2019-01-23T19:18:05Z

On Wed, Jan 23, 2019 at 04:17:30PM +0200, Jani Nikula wrote:
> Can't have:
>
> switch (i) {
> int j;
> case 0:
> /* ... */
> }
>
> because it can't be turned into:
>
> switch (i) {
> int j = 0; /* not valid C */
> case 0:
> /* ... */
> }
>
> but can have e.g.:
>
> switch (i) {
> case 0:
> {
> int j = 0;
> /* ... */
> }
> }
>
> I think Kees' approach of moving such variable declarations to the
> enclosing block scope is better than adding another nesting block.

Another nesting level would be bad, but I think this is OK:

switch (i) {
case 0: {
int j = 0;
/* ... */
}
case 1: {
void *p = q;
/* ... */
}
}

I can imagine Kees' patch might have a bad effect on stack consumption,
unless GCC can be relied on to be smart enough to notice the
non-overlapping liveness of the vriables and use the same stack slots
for both.