[llvm] [InstCombine] Canonicalize more saturated-add variants (PR #100008)

[Rose Silicon via llvm-commits]

RSilicon wrote:

> Your alive proof has
> 
> 
> 
> ```
> 
> define dso_local i8 @src3(i8 %x, i8  %c) {
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> entry:
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>   %noZero = icmp ne i8 %c, 0
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>   call void @llvm.assume(i1 %noZero)
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>   %add = add nuw i8 %x, %c
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>   %c.not = xor i8 %c, -1
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>   %d = add i8 %c.not, 1
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>   %cmp.not = icmp ugt i8 %x, %d
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>   %cond = select i1 %cmp.not, i8 -1, i8 %add
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>   ret i8 %cond
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> }
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> ```
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> 
> 
> The `nuw` flag there says that the add never overflows and thus never saturates. The transform fails without the nuw. There's no check for the nuw in your transform.

x ugt -C ? x + C: -1 always saturates, but nuw means it doesn't, so this would be malformed IR.

https://github.com/llvm/llvm-project/pull/100008