> On Jan 15, 2024, at 10:06 AM, Jakub Jelinek <ja...@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 >
