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Re: On -ftrapv vs. libcalls...
- From: Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>
- To: Steven Bosscher <stevenb at suse dot de>
- Cc: gcc at gcc dot gnu dot org
- Date: Tue, 31 Aug 2004 15:57:11 +0200
- Subject: Re: On -ftrapv vs. libcalls...
- References: <200408221254.28457.stevenb@suse.de>
Hello,
> 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...
my understanding is that with -ftrapv we call the library function that
causes trap on overflow -- on rtl we no longer know whether the operands
of multiplication are unsigned or not, so we cannot emit MULT for it.
What I do not understand is why it is legal to turn the libcall back
into MULT.
Zdenek