On Fri, 2020-06-26 at 23:17 +0200, Ilya Leoshkevich wrote:
> On Fri, 2020-06-26 at 14:27 -0600, Jeff Law wrote:
> > On Thu, 2020-06-04 at 02:37 +0200, Ilya Leoshkevich via Gcc-patches
> > wrote:
> > > Bootstrapped and regtested on x86_64-redhat-linux, ppc64le-redhat-
> > > linux
> > > and s390x-redhat-linux.
> > >
> > >
> > > Linux Kernel (specifically, drivers/leds/trigger/ledtrig-cpu.c)
> > > build
> > > with GCC 10 fails on s390 with "impossible constraint".
> > >
> > > The problem is that jump threading makes __builtin_constant_p lie
> > > when
> > > an expression in question appears to be a constant on a threading
> > > path.
> > What do you mean by this?
> >
> > What should happen is the path where the queried object is constant,
> > builtin_constant_p will return true. On the other path where it is
> > not,
> > builtin_constant_p will return false.
>
> But what if a path ends before we use the `builtin_constant_p` result?
> In the testcase from the patch we have `i = is_active ? 1 : -1`, which
> produces two paths. On each path `builtin_constant_p (i)` returns
> true, however, the asm statement which depends on this being true is
> not on either path and comes only later.
>
> Here are the relevant tree dumps (sorry, a bit verbose, I couldn't make
> them smaller):
>
> gcc/cc1 ../gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c
> -O2 -march=z196 -mzarch -fdump-tree-all-all
>
> Before 106t.thread1:
>
> ;; basic block 2
> if (is_active_2(D) != 0)
> goto <bb 3>; [50.00%]
> else
> goto <bb 4>; [50.00%]
>
> ;; basic block 3
> # iftmp.0_6 = PHI <1(2)>
> _7 = __builtin_constant_pD.1098 (iftmp.0_6);
> if (_7 != 0)
> goto <bb 6>; [50.00%]
> else
> goto <bb 8>; [50.00%]
>
> ;; basic block 4
> # iftmp.0_4 = PHI <-1(2)>
> _12 = __builtin_constant_pD.1098 (iftmp.0_4);
> if (_12 != 0)
> goto <bb 5>; [50.00%]
> else
> goto <bb 8>; [50.00%]
>
> ;; basic block 5
> if (iftmp.0_4 >= -128)
> goto <bb 7>; [20.00%]
> else
> goto <bb 8>; [80.00%]
>
> ;; basic block 6
> if (iftmp.0_6 <= 127)
> goto <bb 7>; [12.00%]
> else
> goto <bb 8>; [88.00%]
>
> ;; basic block 7
> # iftmp.0_13 = PHI <iftmp.0_6(6), iftmp.0_4(5)>
> # .MEM_10 = VDEF <.MEM_3(D)>
> __asm__ __volatile__("asi %0,%1
> " : "ptr" "=Q" MEM[(intD.6 *)&num_active_cpusD.2277] : "val" "i"
> iftmp.0_13, "Q" MEM[(intD.6 *)&num_active_cpusD.2277] : "memory",
> "cc");
> goto <bb 9>; [100.00%]
> ;; succ: 9 [always] count:91268056 (estimated locally)
> (FALLTHRU,EXECUTABLE)
This looks broken already. iftmp.0_13 is not a constant prior to thread1 and
thus, IIUC is not suitable for use in this instruction. Threading just
simplified things, but it's just as broken before threading as it is after
threading.
Jeff
>
