compiler/cmm/CmmLayoutStack.hs¶
Note [Always false stack check]¶
We can optimise stack checks of the form
if ((Sp + x) - y < SpLim) then .. else ..
where are non-negative integer byte offsets. Since we know that SpLim <= Sp (remember the stack grows downwards), this test must yield False if (x >= y), so we can rewrite the comparison to False. A subsequent sinking pass will later drop the dead code. Optimising this away depends on knowing that SpLim <= Sp, so it is really the job of the stack layout algorithm, hence we do it now.
The control flow optimiser may negate a conditional to increase the likelihood of a fallthrough if the branch is not taken. But not every conditional is inverted as the control flow optimiser places some requirements on the predecessors of both branch targets. So we better look for the inverted comparison too.
