Re: HELP: Questions on unshare_expr

[Qing Zhao <qing.zhao@oracle.com>]

> On Jan 15, 2024, at 10:06 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Mon, Jan 15, 2024 at 02:54:26PM +0000, Qing Zhao wrote:
>> So, before gimplification,  when inserting tree node, we don’t need manually
>> add unshare_expr since the gimplification will automatically unshare nodes. 
> 
> There are cases where unshare_expr is needed even then, such as the uses in
> the sanitizer, because code is then modifying suboperands in place later on
> and if things are shared bad things happen.

for my case, it’s in bound sanitizer, and the instrumentation happens 
during “c_genericize”, which seems before gimplfication. 

So,  when adding instrumentation for bound sanitizer, we still need to 
manually unshare expr even it’s before gimpflication?


If trees can be shared until
> they are unshared before gimplification, one doesn't need to worry about it,
> sure.
> 
>> However, during or after gimplfication, when inserting nodes, we should manually
>> add unshare_expr when we put the same “tree” into multiple operands.
> 
> Yes.
> 
>>> Using a SAVE_EXPR avoids redundant code but it also requires
>>> that the SAVE_EXPR uses are ordered.
>> 
>> “Require the SAVE_EXPR uses are ordered”, does this mean that 
>> SAVE_EXPRs for the same node should be in a correct order? Or something else?
> 
> The basic requirement is that SAVE_EXPR is evaluated somewhere in a code
> which dominates all other uses of the SAVE_EXPR.
> Say
> SAVE_EXPR <something_complex>, if (x) use1 (SAVE_EXPR <something_complex>); else use2 (SAVE_EXPR <something_complex>);
> is fine, but
> if (x) use1 (SAVE_EXPR <something_complex>); else use2 (SAVE_EXPR <something_complex>);
> is not.  Because in the latter case, it will be gimplified into evaluating
> the complex expression in the conditional code guarded on if (x != 0), save
> into some temporary variable and then in the else code just use that
> temporary variable, except it is uninitialized then.

Okay, I see.

Is there utility tool to check for any violation of this order? Or I have to manually check the order myself?

Thanks a lot for the help.

Qing
> 
> 	Jakub
>