On -ftrapv vs. libcalls...

[Steven Bosscher <stevenb@suse.de>]

Hi,

Could anyone please help me get some insight into the following
problem.

Consider,

int
mulv(int a, int b)
{
  return a * b;
}

compiled with -ftrapv.  This gives the following code in the .rtl dump
on ia64.

(insn 18 17 19 1 (set (reg:DI 120 out0)
        (reg:DI 345)) -1 (nil)
    (insn_list:REG_LIBCALL 21 (nil)))

(insn 19 18 20 1 (set (reg:DI 121 out1)
        (reg:DI 347)) -1 (nil)
    (nil))

(call_insn/u 20 19 21 1 (parallel [
            (set (reg:DI 8 r8)
                (call (mem:DI (symbol_ref:DI ("__mulvdi3") [flags 0x41]) [0 S8 A64])
                    (const_int 1 [0x1])))
            (clobber (reg:DI 320 b0))
            (clobber (scratch:DI))
            (clobber (scratch:DI))
        ]) -1 (nil)
    (expr_list:REG_EH_REGION (const_int -1 [0xffffffffffffffff])
        (nil))
    (expr_list (use (reg:DI 1 r1))
        (expr_list (use (reg:DI 121 out1))
            (expr_list (use (reg:DI 120 out0))
                (nil)))))

(insn 21 20 22 1 (set (reg:DI 349)
        (reg:DI 8 r8)) -1 (nil)
    (insn_list:REG_RETVAL 18 (expr_list:REG_EQUAL (mult:DI (reg:DI 345)
                (reg:DI 347))
            (nil))))


Note the __mulvdi3 libcall.  At the first opportunity to do so, the
first jump pass, we collapse it to a normal MULT insn:

(insn 16 15 17 0 (set (reg:SI 348 [ b ])
        (mem/i:SI (reg/f:DI 344) [0 b+0 S4 A32])) -1 (nil)
    (nil))

(insn 17 16 21 0 (set (reg:DI 347 [ b ])
        (sign_extend:DI (reg:SI 348 [ b ]))) -1 (nil)
    (nil))

(insn 21 17 22 0 (set (reg:DI 349)
        (mult:DI (reg:DI 345 [ a ])
            (reg:DI 347 [ b ]))) 105 {muldi3} (nil)
    (nil))
(insn 22 21 23 0 (set (reg:SI 339 [ T.0 ])
        (subreg:SI (reg:DI 349) 0)) -1 (nil)
    (nil))

[ this also happens on x86 and amd64, but there the problem is not
  as obvious - apparently ia64 doesn't even have that function in
  libgcc so it causes a link failure without libcall notes.  ]

Without -ftrapv we never emit it, we just always generate a normal
MULT insn, never the libcall:

(insn 14 13 15 1 (set (reg:SI 345)
        (mem/i:SI (reg/f:DI 335 virtual-stack-vars) [0 a+0 S4 A32])) -1 (nil)
    (nil))

(insn 15 14 16 1 (set (reg:SI 346)
        (mem/i:SI (reg/f:DI 344) [0 b+0 S4 A32])) -1 (nil)
    (nil))

(insn 16 15 17 1 (set (reg:SI 339 [ T.0 ])
        (mult:SI (reg:SI 345)
            (reg:SI 346))) -1 (nil)
    (nil))

I'm trying to get rid of libcall notes, so I would really like to know
and understand why we emit a libcall with -ftrapv.  With my patch, we
cannot fold the libcall anymore, so it's a show stopper for removing
libcalls that we emit the (apparently?) unnecessary libcall.

Is there a way to avoid emiting a libcall in this case?

Not emitting the libcall would be good in any case since it would
improve the initially generated RTL with -ftrapv...

Gr.
Steven