Loop body should be emitted in different scope

[Lancern]

In the comments of CIRGenStmt.cpp:

clangir/clang/lib/CIR/CodeGen/CIRGenStmt.cpp

Lines 913 to 918 in 3189c0d

	// https://en.cppreference.com/w/cpp/language/for
	// While in C++, the scope of the init-statement and the scope of
	// statement are one and the same, in C the scope of statement is
	// nested within the scope of init-statement.
	bool useCurrentScope =
	CGM.getASTContext().getLangOpts().CPlusPlus ? true : false;

I believe the comment's statement is actually wrong. The referenced page says:

• The scope of init-statement and the scope of condition are the same.
• The scope of statement and the scope of expression are disjoint and nested within the scope of init-statement and condition.

Thus in C++ the scope of statement should be in a scope nested within the scope of init-statement, but not in the same scope with init-statement.

For the following C++ code:

void test() {
  for (int i = 0; i < 10; ++i) {
    int x = 42;
    use(x);
  }
}

ClangIR currently emits the following CIR:

cir.func @_Z4testv() {
  cir.scope {
    %0 = cir.alloca !s32i, !cir.ptr<!s32i>, ["i", init]
    %1 = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init]
    // ...
    cir.for : cond {
      // ...
    } body {
      // ...
    } step {
      // ...
    }
  }
  cir.return
}

I actually expect it to be:

cir.func @_Z4testv() {
  cir.scope {
    %0 = cir.alloca !s32i, !cir.ptr<!s32i>, ["i", init]
    // ...
    cir.for : cond {
      // ...
    } body {
      cir.scope {
        %1 = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init]
        // ...
      }
    } step {
      // ...
    }
  }
  cir.return
}

Note the difference of the places of the alloca operation for local variable x.

[bcardosolopes]

Makes sense to me!

[Lancern]

The same problem also exists for while loops. For the following C++ code:

void test() {
  while (int x = foo()) {
    int y = foo();
    use(x, y);
  }
}

ClangIR currently emits:

cir.func @_Z4testv() extra(#fn_attr) {
  cir.scope {
    %0 = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init]
    %1 = cir.alloca !s32i, !cir.ptr<!s32i>, ["y", init]
    cir.while {
      // ...
    } do {
      // ...
    }
  }
  cir.return
}

I actually expect it to be:

cir.func @_Z4testv() extra(#fn_attr) {
  cir.scope {
    %0 = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init]
    cir.while {
      // ...
    } do {
      cir.scope {
        %1 = cir.alloca !s32i, !cir.ptr<!s32i>, ["y", init]
        // ...
      }
    }
  }
  cir.return
}

A somehow unrelated issue is the alloca operation for x, which is a variable declared at the condition part of the while loop. The standard says its lifetime should end at the end of every iteration, and upon each iteration it should be re-created as a fresh object, but seems like the generated CIR does not reflect this fact.