[llvm] [InstCombine] simplify `icmp pred x, ~x` (PR #73990)

[Nikita Popov via llvm-commits]

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@@ -4,6 +4,391 @@
 declare void @llvm.assume(i1)
 declare void @barrier()
 declare void @use.i8(i8)
+declare void @use.i1(i1)
+declare void @use.vec.i1(<2 x i1>)
+
+; X comp X^Neg_C tests, signed.
+; X s< X^Neg_C  -->  X s< 0
+define i1 @src_slt(i8 %x, i128 %x.128) {
+; CHECK-LABEL: @src_slt(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 0
+; CHECK-NEXT:    call void @use.i1(i1 [[CMP]])
+; CHECK-NEXT:    [[CMP_128:%.*]] = icmp slt i128 [[X_128:%.*]], 0
+; CHECK-NEXT:    ret i1 [[CMP_128]]
+;
+  %not = xor i8 %x, -1
+  %cmp = icmp slt i8 %x, %not
+  call void @use.i1(i1 %cmp)
+  %not.128 = xor i128 -170141183460469231731687303715884105728, %x.128
+  %cmp.128 = icmp slt i128 %x.128, %not.128
+  ret i1 %cmp.128
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nikic wrote:

I wouldn't combine two tests into one function in this way. It will make proof-checking more expensive and makes it look like a multi use test.

https://github.com/llvm/llvm-project/pull/73990