[InstCombine] Failure to simplify usub.with.overflow+null check of non-null base pointer

[LebedevRI]

Bugzilla Link	43251
Version	trunk
OS	Linux
CC	@RKSimon,@nikic,@rotateright,@vedantk
Fixed by commit(s)	371718, 371921, 372257, 372341, 372912, 372928, 372930

Extended Description

Again comes up in https://reviews.llvm.org/D67122
for

char* test(char& base, unsigned long offset) {
  return &base - offset;
}

We produce

https://godbolt.org/z/luGEju

and fail to simplify further, but we can:

https://rise4fun.com/Alive/Ppo

[LebedevRI]

Actually, we miss even more general fold:
https://rise4fun.com/Alive/UcDx
https://godbolt.org/z/u_vhyU

:S

[LebedevRI]

https://reviews.llvm.org/D67356

[LebedevRI]

Not unexpectedly, there's predicate that requires is-not-zero check
https://rise4fun.com/Alive/A8h
https://godbolt.org/z/8KvkTq

[LebedevRI]

Alright, two more and we're done with this particular pattern.
https://reviews.llvm.org/D67411 https://reviews.llvm.org/D67412
(other than @​usub.with.overflow itself)

[LebedevRI]

r371718, D67356+D67412 remaining here.

[LebedevRI]

All done here i think, other than @​usub.with.overflow if applicable.

[LebedevRI]

Uhm yeah, did not think of a lot instsimplify folds here...

Name: 0
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ult i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = %no_underflow

Name: 1
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ult i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = %not_null

Name: 2
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ult i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = false

Name: 3 ???
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ult i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = or i1 %no_underflow, %not_null

;-------------------------------------------------------------------------------

Name: 4
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ugt i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = %no_underflow

Name: 5
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ugt i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = %not_null

Name: 6
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ugt i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = false

Name: 7 ???
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ugt i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = or i1 %no_underflow, %not_null

;===============================================================================

Name: 8 ???
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ule i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = and i1 %no_underflow, %not_null

Name: 9
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ule i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = true

Name: 10
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ule i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = %not_null

Name: 11
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp ule i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = %no_underflow

;-------------------------------------------------------------------------------

Name: 12 ???
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp uge i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = and i1 %no_underflow, %not_null

Name: 13
  %adjusted = sub i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp uge i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = true

Name: 14
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp uge i8 %base, %offset
  %r = and i1 %no_underflow, %not_null
=>
  %r = %not_null

Name: 15
  %adjusted = sub i8 %base, %offset
  %not_null = icmp eq i8 %adjusted, 0
  %no_underflow = icmp uge i8 %base, %offset
  %r = or i1 %no_underflow, %not_null
=>
  %r = %no_underflow