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From: "vineetg at rivosinc dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/106533] loop distribution not distributing
 inner loop (to memcpy) when perfect loop nest
Date: Fri, 05 Aug 2022 07:32:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106533

Vineet Gupta <vineetg at rivosinc dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vineetg at rivosinc dot com

--- Comment #1 from Vineet Gupta <vineetg at rivosinc dot com> ---
I'm not familiar with actual algorithm of loop distribution, but I debugged=
 and
found the point of divergence.

loop_distribution::execute() loops thru loops_list (cfun, LI_ONLY_INNERMOST=
).
The copy loop 7 (in both the builds) is processed but
prepare_perfect_loop_nest() returns different values

For single copy src loop, it deduces "perfect nesting" and returns outer lo=
op
3. This essentially skips any further distribution of loop 7.

For multi-loop src build, prepare_perfect_loop_nest() exits early as=20
outer->inner =3D=3D loop fails (outer loop 3 has inner pointing to scaling =
loop 10,
the last loop inside it, not 7 which is first). This causes further logic to
eventually distribute it to 0 loop and memcpy.

I'm not sure if this is a bug or intended, hence this report.=