On Wed, Jan 17, 2018 at 09:13:09AM +0100, Peter Zijlstra wrote: > On Tue, Jan 16, 2018 at 09:12:32PM -0600, Josh Poimboeuf wrote: > > On Tue, Jan 16, 2018 at 08:49:17PM +0100, Peter Zijlstra wrote: > > > Subject: objtool: Even more complex static block checks > > > From: Peter Zijlstra <[email protected]> > > > Date: Tue Jan 16 20:17:01 CET 2018 > > > > > > I've observed GCC transform: > > > > > > f() > > > { > > > if (!static_branch_unlikely()) > > > return; > > > > > > static_assert(); > > > A; > > > } > > > > > > g() > > > { > > > f(); > > > } > > > > > > Into: > > > > > > f() > > > { > > > static_assert(); > > > A; > > > } > > > > > > g() > > > { > > > if (static_branch_unlikely()) > > > f(); > > > } > > > > > > Which results in the assertion landing at f+0. The transformation is > > > valid and useful; it avoids a pointless CALL+RET sequence, so we'll > > > have to teach objtool how to deal with this. > > > > > > Do this by marking all CALL destinations with static_call when called > > > from a static_block and non_static_call when called outside a > > > static_block. This allows us to identify functions called exclusively > > > from a static_block and start them with a static_block. > > > > Ew... where'd you place the assertion to trigger this? > > Its the patch I pastebin'ed you earlier, also see below.
Ah, I remembered you mentioning the problem, just forgot you showed me the patch. > > It's late and my brain has already clocked out, so I'll need to revisit > > this tomorrow. But now I'm wondering if my basic block idea would be a > > better way to solve this. > > I would think basic-blocks are inside functions, and this patch goes > across functions, something you'd still need even if you had basic > blocks. Right, but I was thinking the patch would be a lot simpler with basic blocks. > Also, basic blocks are non-trivial because they can overlap. Hm, I thought a basic block only has one entry point and one exit point. How could they overlap? -- Josh
