On Fri, Oct 20, 2017 at 4:03 AM, Sandra Loosemore <san...@codesourcery.com> wrote: > This is the set of nios2 optimization patches that I've previously > mentioned in these threads: > > https://gcc.gnu.org/ml/gcc/2017-10/msg00016.html > https://gcc.gnu.org/ml/gcc-patches/2017-10/msg00957.html > > To give an overview of what this is for.... > > The nios2 backend currently generates quite bad code for memory > accesses with addresses involving symbolic constants. Like a typical > RISC machine, nios2 requires splitting such 32-bit constants into > HIGH/LO_SUM pairs. Currently this happens in expand, and address > expressions involving such constants are always converted to use a > register indirect form. > > One part of the problem is that the backend currently doesn't > recognize that LO_SUM is a legitimate address form (it's register > indirect with a constant offset using the %lo relocation). That's > fixed in these patches. > > A harder problem is that doing the high/lo_sum splitting in expand > inhibits subsequent optimizations. One such problem arises when you > have accesses to multiple fields in a static structure object. Expand > sees this as many (symbol + offset) expressions involving the same > symbol with different constant offsets. What we should be doing in > that case is CSE'ing the symbol address computation rather than > splitting every such expression individually. > > This patch series attacks that problem by deferring splitting to the > split1 pass, which happens after cse and fwprop optimizations. > Deferring the splitting also requires that TARGET_LEGITIMATE_ADDRESS_P > accept these symbolic constant expressions until the splitting takes > place, and that code that might generate 32-bit constants in other > places (e.g., the movsi expander) must not do so after they are > supposed to have been split.
How do other targets handle this situation? Naiively I'd have handled the splitting at reload/LRA time ... (which would make the flag to test reload_completed) There are quite a number of targets using lo_sum but I'm not sure they share the issue with symbolic constants. Otherwise defering the splitting of course looks like the correct thing to do. Richard. > This patch series also includes general improvements to the cost model > to get better CSE results -- in particular, the nios2 backend has been > completely missing an implementation for TARGET_ADDRESS_COST. I also found > that making TARGET_LEGITIMIZE_ADDRESS smarter resulted in better > address cost modeling by the ivopts pass. > > All together, this resulted in about a 7% code size improvement on the > customer-provided test case I was using for tuning purposes. > > Patches in this set are broken down as follows: > > 1: Switch to LRA. > 2: Detect when splitting has been completed. > 3: Add splitters and recognize the new address modes. > 4: Cost model improvements. > 5: Test cases. > > Part 2 is the piece that relates to the discussion linked above. As > implemented, it works fine, but it's maybe not the best design. I'll > hold off on committing the entire set for at least a few days in case > somebody wants to suggest a better solution. > > -Sandra >
