Re: Predictive commoning leads to register to register moves through memory.

[Jeff Law <law at redhat dot com>]

On 08/28/2015 09:43 AM, Simon Dardis wrote:

> Following Jeff's advice[1] to extract more information from GCC, I've
> narrowed the cause down to the predictive commoning pass inserting
> the load in a loop header style basic block. However, the next pass
> in GCC, tree-cunroll promptly removes the loop and joins the loop
> header to the body of the (non)loop. More oddly, disabling
> conditional store elimination pass or the dominator optimizations
> pass or disabling of jump-threading with --param
> max-jump-thread-duplication-stmts=0 nets the above assembly code. Any
> ideas on an approach for this issue?
I'd probably start by looking at the .optimized tree dump in both cases 
to understand the difference, then (most liklely) tracing that through 
the RTL optimizers into the register allocator.

jeff