[llvm] [DAGCombiner] Fold `sub nuw C, x` -> `xor x, C` when C is a mask (PR #192692)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,74 @@
+; RUN: llc -mtriple=x86_64 < %s | FileCheck %s --check-prefix=X86
+
+define i8 @sub_nuw_mask_4bit(i8 %x) {
+; X86-LABEL: sub_nuw_mask_4bit:
+; X86:       xorb $15, %al
+  %r = sub nuw i8 15, %x
+  ret i8 %r
+}
+
+define i8 @sub_nuw_mask_7bit(i8 %x) {
+; X86-LABEL: sub_nuw_mask_7bit:
+; X86:       xorb $127, %al
+  %r = sub nuw i8 127, %x
+  ret i8 %r
+}
+
+define i8 @sub_nuw_allones_i8(i8 %x) {
+; X86-LABEL: sub_nuw_allones_i8:
+; X86:       notb %al
+  %r = sub nuw i8 255, %x
+  ret i8 %r
+}
+
+define i16 @sub_nuw_mask_i16(i16 %x) {
+; X86-LABEL: sub_nuw_mask_i16:
+; X86:       xorl $255, %eax
+  %r = sub nuw i16 255, %x
+  ret i16 %r
+}
+
+define i32 @sub_nuw_mask_i32(i32 %x) {
+; X86-LABEL: sub_nuw_mask_i32:
+; X86:       xorl $65535, %eax
+  %r = sub nuw i32 65535, %x
+  ret i32 %r
+}
+
+define i8 @sub_nuw_nonmask(i8 %x) {
+; X86-LABEL: sub_nuw_nonmask:
+; X86-NOT:   xorb
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arsenm wrote:

-NOT checks aren't the most reliable thing to check, can you use update_llc_test_checks (and add nounwind to all the functions)

https://github.com/llvm/llvm-project/pull/192692