On Mon, Oct 17, 2022 at 9:44 AM Thomas Schwinge <tho...@codesourcery.com> wrote:
>
> Hi!
>
> On 2022-10-11T10:31:37+0200, Aldy Hernandez via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> > When solving 0 = _15 & 1, we calculate _15 as:
> >
> > [irange] int [-INF, -2][0, +INF] NONZERO 0xfffffffe
> >
> > The known value of _15 is [0, 1] NONZERO 0x1 which is intersected with
> > the above, yielding:
> >
> > [0, 1] NONZERO 0x0
> >
> > This eventually gets copied to a _Bool [0, 1] NONZERO 0x0.
> >
> > This is problematic because here we have a bool which is zero, but
> > returns false for irange::zero_p, since the latter does not look at
> > nonzero bits. This causes logical_combine to assume the range is
> > not-zero, and all hell breaks loose.
> >
> > I think we should just normalize a nonzero mask of 0 to [0, 0] at
> > creation, thus avoiding all this.
>
> 1. This commit r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
> "[PR107195] Set range to zero when nonzero mask is 0" broke a GCC/nvptx
> offloading test case:
>
> UNSUPPORTED: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c
> -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O0
> PASS: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c
> -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O2
> (test for excess errors)
> PASS: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c
> -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O2
> execution test
> [-PASS:-]{+FAIL:+}
> libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c
> -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O2
> scan-nvptx-none-offload-rtl-dump mach "SESE regions:.*
> [0-9]+{[0-9]+->[0-9]+(\\.[0-9]+)+}"
>
> Same for C++.
>
> I'll later send a patch (for the test case!) to fix that up.
>
> 2. Looking into this, I found that this
> commit r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
> "[PR107195] Set range to zero when nonzero mask is 0" actually enables a
> code transformation/optimization that GCC apparently has not been doing
> before! I've tried to capture that in the attached
> "Add 'c-c++-common/torture/pr107195-1.c' [PR107195]".
Nice.
>
> Will you please verify that one? In its current '#if 1' configuration,
> it's all-PASS after commit
> r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
> "[PR107195] Set range to zero when nonzero mask is 0", whereas before, we
> get two calls to 'foo', because GCC apparently didnn't understand the
> relation (optimization opportunity) between 'r *= 2;' and the subsequent
> 'if (r & 1)'.
Yeah, that looks correct. We keep better track of nonzero masks.
>
> I've left in the other '#if' variants in case you'd like to experiment
> with these, but would otherwise clean that up before pushing.
>
> Where does one put such a test case?
>
> Should the file be named 'pr107195' or something else?
The aforementioned patch already has:
* gcc.dg/tree-ssa/pr107195-1.c: New test.
* gcc.dg/tree-ssa/pr107195-2.c: New test.
So I would just add a pr107195-3.c test.
>
> Do we scan 'optimized', or an earlier dump?
>
> At '-O1', the actual code transformation is visible already in the 'dom2'
> dump:
>
> <bb 3> [local count: 536870913]:
> gimple_assign <mult_expr, r_7, r_6(D), 2, NULL>
> + gimple_assign <bit_and_expr, _11, r_7, 1, NULL>
> + goto <bb 6>; [100.00%]
>
> - <bb 4> [local count: 1073741824]:
> - # gimple_phi <r_4, r_6(D)(2), r_7(3)>
> + <bb 4> [local count: 536870912]:
> + # gimple_phi <r_4, r_6(D)(2)>
> gimple_assign <bit_and_expr, _2, r_4, 1, NULL>
> gimple_cond <ne_expr, _2, 0, NULL, NULL>
> - goto <bb 5>; [50.00%]
> + goto <bb 5>; [100.00%]
> else
> - goto <bb 6>; [50.00%]
> + goto <bb 6>; [0.00%]
>
> <bb 5> [local count: 536870913]:
> gimple_call <foo, _3, r_4>
> gimple_assign <plus_expr, r_8, _3, r_4, NULL>
>
> <bb 6> [local count: 1073741824]:
> - # gimple_phi <r_5, r_4(4), r_8(5)>
> + # gimple_phi <r_5, r_4(4), r_8(5), r_7(3)>
> gimple_return <r_5>
>
> And, the actual "avoid second call 'foo'" optimization is visiable
> starting 'dom3':
>
> <bb 3> [local count: 536870913]:
> gimple_assign <mult_expr, r_7, r_6(D), 2, NULL>
> + goto <bb 6>; [100.00%]
>
> - <bb 4> [local count: 1073741824]:
> - # gimple_phi <r_4, r_6(D)(2), r_7(3)>
> - gimple_assign <bit_and_expr, _2, r_4, 1, NULL>
> + <bb 4> [local count: 536870912]:
> + gimple_assign <bit_and_expr, _2, r_6(D), 1, NULL>
> gimple_cond <ne_expr, _2, 0, NULL, NULL>
> - goto <bb 5>; [50.00%]
> + goto <bb 5>; [100.00%]
> else
> - goto <bb 6>; [50.00%]
> + goto <bb 6>; [0.00%]
>
> <bb 5> [local count: 536870913]:
> - gimple_call <foo, _3, r_4>
> - gimple_assign <plus_expr, r_8, _3, r_4, NULL>
> + gimple_assign <integer_cst, _3, 0, NULL, NULL>
> + gimple_assign <ssa_name, r_8, r_6(D), NULL, NULL>
>
> <bb 6> [local count: 1073741824]:
> - # gimple_phi <r_5, r_4(4), r_8(5)>
> + # gimple_phi <r_5, r_6(D)(4), r_6(D)(5), r_7(3)>
> gimple_return <r_5>
>
> ..., but I don't know if either of those would be stable/appropriate to
> scan instead of 'optimized'?
IMO, either dom3 or optimized is fine.
Thanks.
Aldy
