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overflow' check
A follow-up for r329011.
This may be changed to produce @llvm.sub.with.overflow in a later patch,
but for now just make things more consistent overall.
A few observations stem from this:
* There does not seem to be a similar one-instruction fold for uadd-overflow
* I'm not sure we'll want to canonicalize `B u> A` as `usub.with.overflow`,
so since the `icmp` here no longer refers to `sub`,
reconstructing `usub.with.overflow` will be problematic,
and will likely require standalone pass (similar to DivRemPairs).
https://rise4fun.com/Alive/Zqs
Name: (A - B) u> A --> B u> A
%t0 = sub i8 %A, %B
%r = icmp ugt i8 %t0, %A
=>
%r = icmp ugt i8 %B, %A
Name: (A - B) u<= A --> B u<= A
%t0 = sub i8 %A, %B
%r = icmp ule i8 %t0, %A
=>
%r = icmp ule i8 %B, %A
Name: C u< (C - D) --> C u< D
%t0 = sub i8 %C, %D
%r = icmp ult i8 %C, %t0
=>
%r = icmp ult i8 %C, %D
Name: C u>= (C - D) --> C u>= D
%t0 = sub i8 %C, %D
%r = icmp uge i8 %C, %t0
=>
%r = icmp uge i8 %C, %D
llvm-svn: 371101
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----------------------------------------
Name: unsigned sub, overflow, v0
%sub = sub i8 %x, %y
%ov = icmp ugt i8 %sub, %x
=>
%agg = usub_overflow i8 %x, %y
%sub = extractvalue {i8, i1} %agg, 0
%ov = extractvalue {i8, i1} %agg, 1
Done: 1
Optimization is correct!
----------------------------------------
Name: unsigned sub, no overflow, v0
%sub = sub i8 %x, %y
%ov = icmp ule i8 %sub, %x
=>
%agg = usub_overflow i8 %x, %y
%sub = extractvalue {i8, i1} %agg, 0
%not.ov = extractvalue {i8, i1} %agg, 1
%ov = xor %not.ov, -1
Done: 1
Optimization is correct!
llvm-svn: 371099
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