[llvm] [InstCombine] Fold `(icmp pred (trunc nuw/nsw X), C)` -> `(icmp pred X, (zext/sext C))` (PR #87935)

[Nikita Popov via llvm-commits]

================
@@ -555,3 +555,184 @@ define i1 @shl1_trunc_sgt4(i32 %a) {
   %r = icmp sgt i16 %t, 4
   ret i1 %r
 }
+
+define i1 @eq_nuw(i32 %x) {
+; CHECK-LABEL: @eq_nuw(
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i32 [[X:%.*]], 123
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp eq i8 %t, 123
+  ret i1 %r
+}
+
+define i1 @ult_nuw(i32 %x) {
+; CHECK-LABEL: @ult_nuw(
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i32 [[X:%.*]], 45
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp ult i8 %t, 45
+  ret i1 %r
+}
+
+define i1 @ule_nuw(i32 %x) {
+; CHECK-LABEL: @ule_nuw(
+; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i32 [[X]], 46
+; CHECK-NEXT:    call void @use(i8 [[T]])
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp ule i8 %t, 45
+  call void @use(i8 %t)
+  ret i1 %r
+}
+
+define i1 @ugt_nuw(i32 %x) {
+; CHECK-LABEL: @ugt_nuw(
+; CHECK-NEXT:    [[R:%.*]] = icmp ugt i32 [[X:%.*]], 12
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp ugt i8 %t, 12
+  ret i1 %r
+}
+
+define i1 @uge_nuw(i32 %x) {
+; CHECK-LABEL: @uge_nuw(
+; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; CHECK-NEXT:    [[R:%.*]] = icmp ugt i32 [[X]], 98
+; CHECK-NEXT:    call void @use(i8 [[T]])
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp uge i8 %t, 99
+  call void @use(i8 %t)
+  ret i1 %r
+}
+
+define i1 @uge_nuw_i48(i48 %x) {
+; CHECK-LABEL: @uge_nuw_i48(
+; CHECK-NEXT:    [[T:%.*]] = trunc nuw i48 [[X:%.*]] to i8
+; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[T]], 98
+; CHECK-NEXT:    call void @use(i8 [[T]])
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i48 %x to i8
+  %r = icmp uge i8 %t, 99
+  call void @use(i8 %t)
+  ret i1 %r
+}
+
+define i1 @sgt_nuw_fail(i32 %x) {
+; CHECK-LABEL: @sgt_nuw_fail(
+; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt i8 [[T]], 12
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nuw i32 %x to i8
+  %r = icmp sgt i8 %t, 12
+  ret i1 %r
+}
+
+define i1 @ne_nsw(i32 %x) {
+; CHECK-LABEL: @ne_nsw(
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[X:%.*]], 123
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t = trunc nsw i32 %x to i8
+  %r = icmp ne i8 %t, 123
+  ret i1 %r
+}
----------------
nikic wrote:

Especially for the nsw case, it would probably good to have some tests with negative numbers, to see that they are correctly sign extended (rather than zero extended).

https://github.com/llvm/llvm-project/pull/87935