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)
;; basic block 8
# iftmp.0_14 = PHI <iftmp.0_4(5), iftmp.0_4(4), iftmp.0_6(6),
iftmp.0_6(3)>
# .MEM_11 = VDEF <.MEM_3(D)>
__asm__ __volatile__("laa %0,%2,%1
" : "old" "=d" old_8, "ptr" "=Q" MEM[(intD.6 *)&num_active_cpusD.2277]
: "val" "d" iftmp.0_14, "Q" MEM[(intD.6 *)&num_active_cpusD.2277] :
"memory", "cc");
`asi` instruction (bb 7) can be used only with constants, and that's
what `__builtin_constant_p` ensures here. For other values `laa`
instruction (block 8) must be used.
106t.thread1 says:
about to thread: path: 2 -> 4, 4 -> 5, 5 -> 6
about to thread: path: 3 -> 4, 4 -> 5, 5 -> 6, 6 -> 7
and as a result produces this:
;; basic block 2
if (is_active_2(D) != 0)
goto <bb 3>; [50.00%]
else
goto <bb 4>; [50.00%]
;; basic block 3
;; basic block 4
# iftmp.0_13 = PHI <-1(2), 1(3)>
# .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");
This code won't compile - `iftmp.0_13` is not a constant. My idea was
that the root cause is that `i` is constant on each threading path,
but is not constant anymore when they merge. But maybe there is a
deeper issue? What would be a better way to fix this?
> > Fix by disallowing __builtin_constant_p on threading paths.
> >
> > gcc/ChangeLog:
> >
> > 2020-06-03 Ilya Leoshkevich <i...@linux.ibm.com>
> >
> > * tree-ssa-threadbackward.c
> > (thread_jumps::profitable_jump_thread_path):
> > Do not allow __builtin_constant_p on a threading path.
> Sorry, this is wrong.
>
> jeff
>
>
