[llvm] [ConstraintElim] Teach checkAndReplaceCondition about samesign (PR #128168)

[Marina Taylor via llvm-commits]

================
@@ -303,3 +303,43 @@ define i1 @ugt_assumed_positive_values(i8 %a, i8 %b) {
 
   ret i1 %result
 }
+
+define i1 @samesign_flipped_signedness_1(i8 %a, i8 %b)  {
+; CHECK-LABEL: @samesign_flipped_signedness_1(
+; CHECK-NEXT:    [[CMP_UGT:%.*]] = icmp ugt i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_UGT]], label [[GREATER:%.*]], label [[EXIT:%.*]]
+; CHECK:       greater:
+; CHECK-NEXT:    ret i1 true
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 false
+;
+  %cmp_ugt = icmp ugt i8 %a, %b
+  br i1 %cmp_ugt, label %greater, label %exit
+
+greater:
+  %cmp_sgt = icmp samesign sgt i8 %a, %b
+  ret i1 %cmp_sgt
+
+exit:
+  ret i1 false
+}
+
+define i1 @samesign_flipped_signedness_2(i32 %a) {
+; CHECK-LABEL: @samesign_flipped_signedness_2(
+; CHECK-NEXT:    [[CMP_ULT:%.*]] = icmp samesign ult i32 [[A:%.*]], 65
+; CHECK-NEXT:    br i1 [[CMP_ULT]], label [[FOR_COND_PREHEADER:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       for.cond.preheader:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i1 true
+;
+  %cmp_ult = icmp samesign ult i32 %a, 65
+  br i1 %cmp_ult, label %for.cond.preheader, label %if.else
+
+for.cond.preheader:
+  ret i1 false
+
+if.else:
+  %cmp_ugt = icmp samesign ugt i32 %a, -65
+  ret i1 %cmp_ugt
----------------
citymarina wrote:

Putting together the recent comments:
* Exactly one of the icmps should have samesign
* samesign only goes with unsigned
* Want a dominating samesign test

that lead me to write:
```
define i1 @implied_condition_ugt_sgt(i8 %a, i8 %b)  {
  %cmp_ugt = icmp samesign ugt i8 %a, %b
  br i1 %cmp_ugt, label %greater, label %exit

greater:
  %cmp_sgt = icmp sgt i8 %a, %b
  ret i1 %cmp_sgt

exit:
  ret i1 false
}
```
but in this code, `greater:` is already optimized to `ret i1 true` even without this patch. Have I misunderstood some part?


https://github.com/llvm/llvm-project/pull/128168