Re: [PATCH V2 2/8] bpf: new GCC port

[jose dot marchesi at oracle dot com (Jose E. Marchesi)]

    > [...]
    > +;; Division
    > +(define_insn "div<AM:mode>3"
    > +  [(set (match_operand:AM 0 "register_operand" "=r,r")
    > +        (div:AM (match_operand:AM 1 "register_operand" " 0,0")
    > +                (match_operand:AM 2 "reg_or_imm_operand" "r,I")))]
    > +  ""
    > +  "div<msuffix>\t%0,%2"
    > +  [(set_attr "type" "<mtype>")])
    > +
    > +(define_insn "udiv<AM:mode>3"
    > +  [(set (match_operand:AM 0 "register_operand" "=r,r")
    > +        (div:AM (match_operand:AM 1 "register_operand" " 0,0")
    > +                (match_operand:AM 2 "reg_or_imm_operand" "r,I")))]
    > +  ""
    > +  "div<msuffix>\t%0,%2"
    > +  [(set_attr "type" "<mtype>")])
    
    div and udiv are two different operations.  I don't see how we can
    use the same eBPF instruction for both.  The rtl for udiv should also
    use the udiv rtx code.

    > +;;; Modulus
    > +(define_insn "mod<AM:mode>3"
    > +  [(set (match_operand:AM 0 "register_operand" "=r,r")
    > +        (mod:AM (match_operand:AM 1 "register_operand" " 0,0")
    > +                (match_operand:AM 2 "reg_or_imm_operand" "r,I")))]
    > +  ""
    > +  "mod<msuffix>\t%0,%2"
    > +  [(set_attr "type" "<mtype>")])
    > +
    > +(define_insn "umod<AM:mode>3"
    > +  [(set (match_operand:AM 0 "register_operand" "=r,r")
    > +        (mod:AM (match_operand:AM 1 "register_operand" " 0,0")
    > +                (match_operand:AM 2 "reg_or_imm_operand" "r,I")))]
    > +  ""
    > +  "mod<msuffix>\t%0,%2"
    > +  [(set_attr "type" "<mtype>")])
    
    Same here, with umod for the rtx code.

Oh dear the signed division...  during development I just made both
signed and unsigned flavors to use the same instructions, then forgot to
change it.

Why did I do that?  Because eBPF does not provide instructions for doing
_signed_ division, nor signed remainder: both `div' and `mod' perform
unsigned arithmetic.

clang/llvm ICEs whenever it finds signed division in a C program:

   $ clang -target bpf foo.c
   Error: Unsupport signed division for DAG: t17: i64 = sdiv t15, t16Please convert to unsigned div/mod.
   fatal error: error in backend: Cannot select: t17: i64 = sdiv t15, t16

For GCC I much prefer for the compiler to generate funcalls instead, to
__divdi3/__divsi3/__moddi3/__modsi3 or the like, even if nothing is
providing implementations for these functions (yet.)

So I just defined the u{div,mod}MODE3 patterns in bpf.md (yes this time
using the right rtl opcode :P) and removed the insns for signed
operations.

Thanks for noticing this!