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Re: RFC: Patch for switch elimination (PR 54742)

On 08/13/14 14:55, Sebastian Pop wrote:
Steve Ellcey wrote:
+/* This file implements an optimization where, when a variable is set
+   to a constant value and there is a path that leads from that definition
+   to a switch statement that uses that variable as its controlling expression
+   we duplicate the blocks on this path and change the jump to the switch
+   statement with a direct jump to the label of the switch block that control
+   would goto based on the value of the variable.  This can come up in
+   loops/switch statements that implement state machines.

Can you explain why the jump-threading pass cannot do this?  Why do we need
another pass to handle a corner case that jump-thread does not catch yet?
I'm pretty sure jump threading *could* handle it, but after looking at the full testcase when it was posted, I'm not sure it's *wise* to handle this in jump threading.

The core issue is we have to look even deeper into the CFG than was originally envisioned and do quite a bit more block duplication than was originally envisioned. That's going to have a notable compile-time cost (and to a lesser extent issues around codesize).

It's unfortunate that the testcase when we first looked at this was over-simplified and not actually representative of what happens in Coremark. Had I seen the Coremark testcase, I probably wouldn't have suggested we tackle the problem in jump threading and the extensions I made to jump threading would _not_ have included aggressively following backedges in the CFG.

You'll note in a separate thread Steve and I discussed this during Cauldron and it was at my recommendation Steve resurrected his proof of concept plugin and started beating it into shape.


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