[llvm] [DeadStoreElimination] Optimize tautological assignments (PR #75744)

[Nikita Popov via llvm-commits]

================
@@ -795,3 +795,369 @@ join:
   store i8 %v, ptr %q, align 1
   ret void
 }
+
+; Dominating condition implies value already exists, optimize store
+define void @remove_tautological_store_eq(ptr %x) {
+; CHECK-LABEL: @remove_tautological_store_eq(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL:%.*]] = load i32, ptr [[X:%.*]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[VAL]], 4
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_EQ:%.*]], label [[END:%.*]]
+; CHECK:       if.eq:
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %val = load i32, ptr %x, align 4
+  %cmp = icmp eq i32 %val, 4
+  br i1 %cmp, label %if.eq, label %end
+
+if.eq:
+  store i32 4, ptr %x, align 4
+  br label %end
+
+end:
+  ret void
+}
+
+; Dominating condition implies value already exists, optimize store
+define void @remove_tautological_store_var(ptr %x, ptr %y) {
+; CHECK-LABEL: @remove_tautological_store_var(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VALX:%.*]] = load i32, ptr [[X:%.*]], align 4
+; CHECK-NEXT:    [[VALY:%.*]] = load i32, ptr [[Y:%.*]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[VALX]], [[VALY]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_EQ:%.*]], label [[END:%.*]]
+; CHECK:       if.eq:
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %valx = load i32, ptr %x, align 4
+  %valy = load i32, ptr %y, align 4
+  %cmp = icmp eq i32 %valx, %valy
+  br i1 %cmp, label %if.eq, label %end
+
+if.eq:
+  store i32 %valy, ptr %x, align 4
+  br label %end
+
+end:
+  ret void
+}
+
+; Dominating condition implies value already exists, optimize store
+define void @remove_tautological_store_ne(ptr %x) {
+; CHECK-LABEL: @remove_tautological_store_ne(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL:%.*]] = load i32, ptr [[X:%.*]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[VAL]], 4
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_NE:%.*]], label [[END:%.*]]
+; CHECK:       if.ne:
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    store i32 4, ptr [[X]], align 4
+; CHECK-NEXT:    ret void
+;
+entry:
+  %val = load i32, ptr %x, align 4
+  %cmp = icmp ne i32 %val, 4
+  br i1 %cmp, label %if.ne, label %end
+
+if.ne:
+  br label %end
+
+end:
+  store i32 4, ptr %x, align 4
+  ret void
+}
+
+; Dominating condition implies value already exists, optimize store
+; Optimizes unordered atomic stores
+define void @remove_tautological_store_atomic_unordered(ptr %x) {
+; CHECK-LABEL: @remove_tautological_store_atomic_unordered(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL:%.*]] = load i32, ptr [[X:%.*]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[VAL]], 4
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_EQ:%.*]], label [[END:%.*]]
+; CHECK:       if.eq:
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %val = load i32, ptr %x, align 4
+  %cmp = icmp eq i32 %val, 4
+  br i1 %cmp, label %if.eq, label %end
+
+if.eq:
+  store atomic i32 4, ptr %x unordered, align 4
+  br label %end
+
+end:
+  ret void
+}
+
+; Dominating condition implies value already exists, optimize store
----------------
nikic wrote:

For the negative tests I'd drop this "optimize store" comment, it's confusing for tests that shouldn't actually optimize. Only leave the "Should not" comment below.

https://github.com/llvm/llvm-project/pull/75744