[llvm] [InstCombine] fold icmp of select with invertible shl (PR #147182)

[via llvm-commits]

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@@ -835,3 +835,91 @@ define i1 @discr_eq_constantexpr(ptr %p) {
   %cmp = icmp eq i64 %sub, -1
   ret i1 %cmp
 }
+
+define i1 @shl_nsw_eq_simplify_zero_to_self(i8 %a, i1 %cond) {
+; CHECK-LABEL: @shl_nsw_eq_simplify_zero_to_self(
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i1 [[COND:%.*]] to i8
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[A:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %shl = shl nsw i8 %a, 3
+  %sel = select i1 %cond, i8 8, i8 0
+  %cmp = icmp eq i8 %shl, %sel
+  ret i1 %cmp
+}
+
+define i1 @shl_nsw_eq(i8 %a, i1 %cond) {
+; CHECK-LABEL: @shl_nsw_eq(
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[COND:%.*]], i8 1, i8 -15
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[A:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %shl = shl nsw i8 %a, 3
+  %sel = select i1 %cond, i8 8, i8 136
+  %cmp = icmp eq i8 %shl, %sel
+  ret i1 %cmp
+}
+
+define i1 @shl_nuw_eq(i8 %a, i1 %cond) {
+; CHECK-LABEL: @shl_nuw_eq(
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[COND:%.*]], i8 1, i8 17
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[A:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %shl = shl nuw i8 %a, 3
+  %sel = select i1 %cond, i8 8, i8 136
+  %cmp = icmp eq i8 %shl, %sel
+  ret i1 %cmp
+}
+
+define i1 @shl_nsw_failed_to_simplify(i8 %a, i1 %cond) {
+; CHECK-LABEL: @shl_nsw_failed_to_simplify(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[A:%.*]], 1
+; CHECK-NEXT:    [[NOT_COND:%.*]] = xor i1 [[COND:%.*]], true
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actinks wrote:

failed to simplify but already get folded in a different way @nikic 

https://github.com/llvm/llvm-project/pull/147182