[llvm] [InstCombine] Fold out-of-range bits for squaring signed integers (PR #153484)

[Nikita Popov via llvm-commits]

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@@ -423,3 +423,35 @@ define i64 @smear_set_bit_different_dest_type_wider_dst(i32 %x) {
   %s = sext i8 %a to i64
   ret i64 %s
 }
+
+; Test known bits for (sext i8 x) * (sext i8 x)
+
+define i1 @sext_square_bit30(i8 %x) {
+; CHECK-LABEL: @sext_square_bit30(
+; CHECK:  ret i1 false
+  %sx = sext i8 %x to i32
+  %mul = mul nsw i32 %sx, %sx
+  %and = and i32 %mul, 1073741824 ; 1 << 30
+  %cmp = icmp ne i32 %and, 0
+  ret i1 %cmp
+}
+
+define i1 @sext_square_bit15(i8 %x) {
+; CHECK-LABEL: @sext_square_bit15(
+; CHECK:  ret i1 false
+  %sx = sext i8 %x to i32
+  %mul = mul nsw i32 %sx, %sx
+  %and = and i32 %mul, 32768 ; 1 << 15
+  %cmp = icmp ne i32 %and, 0
+  ret i1 %cmp
+}
+
+define i1 @sext_square_bit14(i8 %x) {
+; CHECK-LABEL: @sext_square_bit14(
+; CHECK-NOT: ret i1 false
+  %sx = sext i8 %x to i32
+  %mul = mul nsw i32 %sx, %sx
+  %and = and i32 %mul, 16384 ; 1 << 14
+  %cmp = icmp ne i32 %and, 0
+  ret i1 %cmp
+}
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nikic wrote:

Though I don't really get why this case is folding. We should have TyBits = 32, SignBits = 9, OutValidBits = 16, OutSignBits = 17. So I would not expect bit 1<<14 to be set to zero.

https://github.com/llvm/llvm-project/pull/153484