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On 03/12/2016 04:10 AM, Richard Biener wrote:
Undefined? Most likely. But we still have to do something sensible. As Jakub noted, a user could create the problematic code just as easily as DCE/DSE, so IRA probably needs to be tolerant of this situation.On March 12, 2016 10:29:40 AM GMT+01:00, Jakub Jelinek <jakub@redhat.com> wrote:On Sat, Mar 12, 2016 at 07:37:25PM +1030, Alan Modra wrote:I believe Alan's point is DSE deleted the assignment to x whichcan't beright as long as we've left in goto *&x. The goto *&x should be a use of x and thus should have kept theassignmentlive.Right, I wasn't trying to say that ira.c:indirect_jump_optimize is OK. It needs the patch I posted or perhaps even better a test of DF_REF_INSN_INFO rather than !DF_REF_IS_ARTIFICIAL (simply becausetheflag test is reading another field, and we need to read DF_REF_INSN_INFO anyway).Ok, that was my point. BTW, DSE isn't the only one that deletes x = 0; cddce deletes it too. -fno-tree-dse -fno-tree-dce preserves it till expansion.GIMPLE_GOTO doesn't have VOPs and I don't think that we'd want VUSEs on all gotos. But just having them on indirect gotos would be inconsistent. I believe the code is undefined anyway and out of scope of a reasonable QOI.
So it seems like you're suggesting we leave DCE/DSE alone (declaring this usage undefined) and fix IRA to be tolerant, right?
Given that executable stacks are a huge security hole, I'd be willing to go out on a limb and declare that undefined as well. It's not as clear cut, but that's the argument I'd make.Using alloca to create/jump to code on the stack should work (we might transform that into a decl though).
And yes, I realize that goes in opposition to what GCC has allowed for 20+ years. I still think it'd be the right thing to do.
jeff
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