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The fold 'A - (A & (B - 1))' -> 'A & (0 - B)'
added in 8dab0a4a7d691f2704f1079538e0ef29548db159
is too specific. It should/can just be 'A - (A & B)' -> 'A & (~B)'
Even if we don't manage to fold `~` into B,
we have likely formed `ANDN` node.
Also, this way there's less similar-but-duplicate folds.
Name: X - (X & Y) -> X & (~Y)
%o = and i32 %X, %Y
%r = sub i32 %X, %o
=>
%n = xor i32 %Y, -1
%r = and i32 %X, %n
https://rise4fun.com/Alive/kOUl
See
https://bugs.llvm.org/show_bug.cgi?id=44448
https://reviews.llvm.org/D71499
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While we do manage to fold integer-typed IR in middle-end,
we can't do that for the main motivational case of pointers.
There is @llvm.ptrmask() intrinsic which may or may not be helpful,
but i'm not sure it is fully considered canonical yet,
not everything is fully aware of it likely.
https://rise4fun.com/Alive/ZVdp
Name: ptr - (ptr & (alignment-1)) -> ptr & (0 - alignment)
%mask = add i64 %alignment, -1
%bias = and i64 %ptr, %mask
%r = sub i64 %ptr, %bias
=>
%highbitmask = sub i64 0, %alignment
%r = and i64 %ptr, %highbitmask
See
https://bugs.llvm.org/show_bug.cgi?id=44448
https://reviews.llvm.org/D71499
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https://rise4fun.com/Alive/ZVdp
Name: ptr - (ptr & (alignment-1)) -> ptr & (0 - alignment)
%mask = add i64 %alignment, -1
%bias = and i64 %ptr, %mask
%r = sub i64 %ptr, %bias
=>
%highbitmask = sub i64 0, %alignment
%r = and i64 %ptr, %highbitmask
The main motivational pattern involes pointer-typed values,
so this transform can't really be done in middle-end.
See
https://bugs.llvm.org/show_bug.cgi?id=44448
https://reviews.llvm.org/D71499
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