Roman Lebedev
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9e97862e1f
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[Codegen] TargetLowering::prepareUREMEqFold(): x u% C1 ==/!= C2 (PR35479)
Summary:
The current lowering is:
```
Name: (X % C1) == C2 -> X * C3 <= C4 || false
Pre: (C2 == 0 || C1 u<= C2) && (C1 u>> countTrailingZeros(C1)) * C3 == 1
%zz = and i8 C3, 0 ; trick alive into making C3 avaliable in precondition
%o0 = urem i8 %x, C1
%r = icmp eq i8 %o0, C2
=>
%zz = and i8 C3, 0 ; and silence it from complaining about said reg
%C4 = -1 /u C1
%n0 = mul i8 %x, C3
%n1 = lshr i8 %n0, countTrailingZeros(C1) ; rotate right
%n2 = shl i8 %n0, ((8-countTrailingZeros(C1)) %u 8) ; rotate right
%n3 = or i8 %n1, %n2 ; rotate right
%is_tautologically_false = icmp ule i8 C1, C2
%C4_fixed = select i1 %is_tautologically_false, i8 -1, i8 %C4
%res = icmp ule i8 %n3, %C4_fixed
%r = xor i1 %res, %is_tautologically_false
```
https://rise4fun.com/Alive/2xC
https://rise4fun.com/Alive/jpb5
However, we can support non-tautological cases `C1 u> C2` too.
Said handling consists of two parts:
* `C2 u<= (-1 %u C1)`. It just works. We only have to change `(X % C1) == C2` into `((X - C2) % C1) == 0`
```
Name: (X % C1) == C2 -> (X - C2) * C3 <= C4 iff C2 u<= (-1 %u C1)
Pre: (C1 u>> countTrailingZeros(C1)) * C3 == 1 && C2 u<= (-1 %u C1)
%zz = and i8 C3, 0 ; trick alive into making C3 avaliable in precondition
%o0 = urem i8 %x, C1
%r = icmp eq i8 %o0, C2
=>
%zz = and i8 C3, 0 ; and silence it from complaining about said reg
%C4 = (-1 /u C1)
%n0 = sub i8 %x, C2
%n1 = mul i8 %n0, C3
%n2 = lshr i8 %n1, countTrailingZeros(C1) ; rotate right
%n3 = shl i8 %n1, ((8-countTrailingZeros(C1)) %u 8) ; rotate right
%n4 = or i8 %n2, %n3 ; rotate right
%is_tautologically_false = icmp ule i8 C1, C2
%C4_fixed = select i1 %is_tautologically_false, i8 -1, i8 %C4
%res = icmp ule i8 %n4, %C4_fixed
%r = xor i1 %res, %is_tautologically_false
```
https://rise4fun.com/Alive/m4P
https://rise4fun.com/Alive/SKrx
* `C2 u> (-1 %u C1)`. We also have to change `(X % C1) == C2` into `((X - C2) % C1) == 0`,
and we have to decrement C4:
```
Name: (X % C1) == C2 -> (X - C2) * C3 <= C4 iff C2 u> (-1 %u C1)
Pre: (C1 u>> countTrailingZeros(C1)) * C3 == 1 && C2 u> (-1 %u C1)
%zz = and i8 C3, 0 ; trick alive into making C3 avaliable in precondition
%o0 = urem i8 %x, C1
%r = icmp eq i8 %o0, C2
=>
%zz = and i8 C3, 0 ; and silence it from complaining about said reg
%C4 = (-1 /u C1)-1
%n0 = sub i8 %x, C2
%n1 = mul i8 %n0, C3
%n2 = lshr i8 %n1, countTrailingZeros(C1) ; rotate right
%n3 = shl i8 %n1, ((8-countTrailingZeros(C1)) %u 8) ; rotate right
%n4 = or i8 %n2, %n3 ; rotate right
%is_tautologically_false = icmp ule i8 C1, C2
%C4_fixed = select i1 %is_tautologically_false, i8 -1, i8 %C4
%res = icmp ule i8 %n4, %C4_fixed
%r = xor i1 %res, %is_tautologically_false
```
https://rise4fun.com/Alive/d40
https://rise4fun.com/Alive/8cF
I believe this concludes `x u% C1 ==/!= C2` lowering.
In fact, clang is may now be better in this regard than gcc:
as it can be seen from `@t32_6_4` test, we do lower `x % 6 == 4`
via this pattern, while gcc does not: https://godbolt.org/z/XNU2z9
And all the general alive proofs say this is legal.
And manual checking agrees: https://rise4fun.com/Alive/WA2
Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=35479 | PR35479 ]].
Reviewers: RKSimon, craig.topper, spatel
Reviewed By: RKSimon
Subscribers: nick, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70053
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2019-11-22 15:22:42 +03:00 |
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