On 7/29/19 9:50 AM, Jakub Jelinek wrote: > On Tue, Jul 23, 2019 at 04:26:24AM +0000, JiangNing OS wrote: >> --- a/gcc/ChangeLog >> +++ b/gcc/ChangeLog >> @@ -1,3 +1,9 @@ >> +2019-07-22 Jiangning Liu <jiangning....@amperecomputing.com> >> + >> + PR middle-end/91195 >> + * tree-ssa-phiopt.c (cond_store_replacement): Work around >> + -Wmaybe-uninitialized warning. >> + >> 2019-07-22 Stafford Horne <sho...@gmail.com> >> >> * config/or1k/or1k.c (or1k_expand_compare): Check for int before >> diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c >> index b64bde695f4..7a86007d087 100644 >> --- a/gcc/tree-ssa-phiopt.c >> +++ b/gcc/tree-ssa-phiopt.c >> @@ -2240,6 +2240,14 @@ cond_store_replacement (basic_block middle_bb, >> basic_block join_bb, >> tree base = get_base_address (lhs); >> if (!auto_var_p (base) || TREE_ADDRESSABLE (base)) >> return false; >> + >> + /* The transformation below will inherently introduce a memory load, >> + for which LHS may not be initialized yet if it is not in NOTRAP, >> + so a -Wmaybe-uninitialized warning message could be triggered. >> + Since it's a bit hard to have a very accurate uninitialization >> + analysis to memory reference, we disable the warning here to avoid >> + confusion. */ >> + TREE_NO_WARNING (lhs) = 1; > > I don't like this, but not for the reasons Martin stated, we use > TREE_NO_WARNING not just when we've emitted warnings, but in many places > when we've done something that might trigger false positives. > Yes, it would be nice to do it more selectively. > > The problem I see with the above though is that lhs might very well be > a decl, and setting TREE_NO_WARNING on it then doesn't affect only the > hoisted load, but also all other code that refers to the decl. LHS is restricted to just MEM_REF, ARRAY_REF and COMPONENT_REF. We'd be setting the NO_WARNING bits on the toplevel expression, but not on anything shared like a _DECL node.
So what we're losing here would be things like out of bounds array checks on the LHS, so it still sucks. > > If the TREE_NO_WARNING bit is set on something that isn't shareable, that is > fine with me, like a MEM_REF, TARGET_MEM_REF or handled component. If lhs > is a decl, can we force a MEM_REF around it (and won't we fold it back to > the decl?)? Or perhaps better, can we gimple_set_no_warning on the load > stmt instead? We have the toplevel statement, so that might be worth a try as well. jeff
