[llvm] [ConstraintElim] decompose subtract with guard as precondition (PR #209615)

[Florian Hahn via llvm-commits]

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@@ -239,3 +239,37 @@ if.end:                                           ; preds = %entry
 
 
 declare void @use(i1)
+
+; A `sub` without nsw/nuw is decomposed in the unsigned system under the
+; recorded precondition (Op1 u<= Op0). Given `%b u<= %a`, `%a - %b u<= %a` is
+; therefore known and the check folds to true. Without the precondition (the
+; check before the branch) it must NOT fold.
+define i1 @sub_no_wrap_flags_ule(i8 %a, i8 %b) {
+; CHECK-LABEL: @sub_no_wrap_flags_ule(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SUB_NEG:%.*]] = sub i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[C_NEG:%.*]] = icmp ule i8 [[SUB_NEG]], [[A]]
+; CHECK-NEXT:    call void @use(i1 [[C_NEG]])
+; CHECK-NEXT:    [[PRECOND:%.*]] = icmp ule i8 [[B]], [[A]]
+; CHECK-NEXT:    br i1 [[PRECOND]], label [[THEN:%.*]], label [[ELSE:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    [[SUB:%.*]] = sub i8 [[A]], [[B]]
+; CHECK-NEXT:    ret i1 true
+; CHECK:       else:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %sub.neg = sub i8 %a, %b
+  %c.neg = icmp ule i8 %sub.neg, %a
+  call void @use(i1 %c.neg)
+  %precond = icmp ule i8 %b, %a
+  br i1 %precond, label %then, label %else
+
+then:
+  %sub = sub i8 %a, %b
+  %c = icmp ule i8 %sub, %a
+  ret i1 %c
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fhahn wrote:

you can make this more compact by avoiding CFG and using `call void @llvm.assume(i1 %precond)`. same for test above

https://github.com/llvm/llvm-project/pull/209615