Hi Matt, On Wed, 15 Nov 2017, Maciej W. Rozycki wrote:
> Can you send me .i output from the offending source along with GCC > options used to make .o output (use `V=1' with `make' if needed)? I'll > check if my hypothesis is right or find the actual cause otherwise. Thanks for the pieces requested. I can see what is going on here: the problem is the source is built with no optimisation enabled. Consequently GAS does not schedule delay slots itself. I wasn't aware we had this problem with kernel build flags -- it certainly affects more than just this piece of code. For GAS to schedule delay slots it has to be passed -O2 by the GCC driver. The driver in turn will pass -O2 to GAS whenever at least -O has been used. By default -O1 is passed, which only enables the removal of unnecessary NOPs, i.e. those that have been inserted precautionarily for execution hazard avoidance in the assembly pass and in the end turned out unnecessary and can be optionally removed in the relaxation pass. NB GAS's default is actually -O2, i.e. when invoked directly, however this is overridden by the GCC driver as described. My recommendation would be using just the same CFLAGS for assembly sources that are used for C language sources; the GCC driver will silently filter out options that are irrelevant and interpret all the options that do have relevance. This may be important for various `-f<foo>' and `-m<bar>' options which may sometimes affect assembly generation and, for that matter, multilib selection. This also means all CFLAGS should be passed to the linker as well, because again, they may affect linker options. So if you stick say `-O2' with your compiler invocation, for the purpose of this consideration possibly just like this: $ make AFLAGS_genex.o=-O2 arch/mips/kernel/genex.o then you should get the delay slot in question (and any other ones) scheduled. Indeed with this build recipe applied I can already see a small code size reduction in `genex.o' even without your change: $ size arch/mips/kernel/genex-?.o text data bss dec hex filename 8236 48 0 8284 205c arch/mips/kernel/genex-0.o 8204 48 0 8252 203c arch/mips/kernel/genex-1.o $ HTH, Maciej
