Re: RISC-V sibcall optimization with save-restore

[Jim Wilson <jimw@sifive.com>]

On 3/20/19 5:25 AM, Paulo Matos wrote:
> I am working on trying to get RISC-V 32 emitting sibcalls even in the
> present of `-msave-restore`, for a client concerned with generated code
> size.

This won't work unless you define a new set of restore functions.  The 
current ones restore the return address from the stack and return, which 
is wrong if you want to do a sibcall.  This is why we tail call (jump 
to) the restore functions, because the actual function return is in the 
restore functions.  You will need a new set of restore functions that 
restore regs without restoring the ra.  You then probably also then need 
other cascading changes to make this work.

The new set of restore functions will then increase code size a bit 
offsetting the gain you get from using them.  You would have to have 
enough sibling calls that can use -msave-restore to make this 
worthwhile.  It isn't clear if this would be a win or not.

> I thought I was on the right path until I noticed that the CFG is messed
> up because of assumptions related to emission of sibcall instead of a
> libcall until the epilogue is expanded. During the pro_and_epilogue pass
> I get an emergency dump and a segfault:
> gcc/gcc/testsuite/gcc.target/riscv/save-restore-1.c:11:1: error: in
> basic block 2:
> gcc/gcc/testsuite/gcc.target/riscv/save-restore-1.c:11:1: error: flow
> control insn inside a basic block
> (jump_insn 24 23 6 2 (parallel [
>              (return)
>              (use (reg:SI 1 ra))
>              (const_int 0 [0])
>          ]) "gcc/gcc/testsuite/gcc.target/riscv/save-restore-1.c":11:1 -1
>       (nil))

If you look at the epilogue code, you will see that it emits a regular 
instruction which hides the call to the restore routine, and then it 
emits a special fake return insn that doesn't do anything.  You can just 
stop emitting the special fake return insn in this case.  This of course 
assumes that you have a new set of restore functions that actually 
return the caller, instead of the caller's parent.

One of the issues with -msave-restore is that the limited offset ranges 
of calls and branches means that if you don't have a tiny program then 
each save/restore call/jump is probably an auipc/lui plus the call/tail, 
which limits the code size reduction you get from using it.  If you can 
control where the -msave-restore routines are placed in memory, then 
putting them near address 0, or near the global pointer address, will 
allow linker relaxation to optimize these calls/jumps to a single 
instruction.  This will probably help more than trying to get it to work 
with sibling calls.

If you can modify the hardware, you might try adding load/store multiple 
instructions and using that instead of the -msave-restore option.  I 
don't know if anyone has tried this yet, but it would be an interesting 
experiment that might result in smaller code size.

Jim