[C11] Correct global atomic pointer initialization from an integer constant

[aaron.ballman]

Summary

This is a follow-up to 2e275e24355cb224981f9beb2b026a3169fc7232 and 1395cde24b3641e284bb1daae7d56c189a2635e3 which corrects a missed case: initializing an _Atomic(T *) from a null pointer constant in the form of the integer literal 0.

(Note: a release note is unnecessary for this change as it's correcting an issue introduced during this cycle.)

Fixes https://github.com/llvm/llvm-project/issues/63550

[efriedma]

The testcase doesn't actually trigger the assertion... but I guess it still tests the output, so that's probably okay? Maybe add a case static _Atomic(int *) glob_pointer_from_long = 0LL; just to be sure.

Otherwise LGTM

(The "VarSize == CstSize" assertion has been catching a surprising number of cases of bad codegen; I'm happy I spent the time to add it.)

[aaron.ballman]

In D154284#4467154, @efriedma wrote:

The testcase doesn't actually trigger the assertion... but I guess it still tests the output, so that's probably okay? Maybe add a case static _Atomic(int *) glob_pointer_from_long = 0LL; just to be sure.

Thanks for catching that! I added an additional test that doesn't use static, as that was relevant to reproducing the original issue: https://godbolt.org/z/cf4Wzeo3z

Otherwise LGTM

(The "VarSize == CstSize" assertion has been catching a surprising number of cases of bad codegen; I'm happy I spent the time to add it.)

Yeah, this is a handy assertion!

clang/lib/AST/ExprConstant.cpp

return This ? EvaluateInPlace(Result, Info, *This, &VIE)
                 : Evaluate(Result, Info, &VIE);
   }
 
   bool VisitCastExpr(const CastExpr *E) {
     switch (E->getCastKind()) {
     default:
       return ExprEvaluatorBaseTy::VisitCastExpr(E);
     case CK_NullToPointer:
+      VisitIgnoredValue(E->getSubExpr());
+      return ZeroInitialization(E);
     case CK_NonAtomicToAtomic:
       return This ? EvaluateInPlace(Result, Info, *This, E->getSubExpr())
                   : Evaluate(Result, Info, E->getSubExpr());
     }
   }
 };
 } // end anonymous namespace
 

clang/test/CodeGen/atomic.c

 // RUN: %clang_cc1 %s -emit-llvm -o - -triple=i686-apple-darwin9 | FileCheck %s
 
+// CHECK: @[[NONSTATIC_GLOB_POINTER_FROM_INT:.+]] = global ptr null
 // CHECK: @[[GLOB_POINTER:.+]] = internal global ptr null
+// CHECK: @[[GLOB_POINTER_FROM_INT:.+]] = internal global ptr null
 // CHECK: @[[GLOB_INT:.+]] = internal global i32 0
 // CHECK: @[[GLOB_FLT:.+]] = internal global float {{[0e\+-\.]+}}, align
 
 int atomic(void) {
   // non-sensical test for sync functions
   int old;
   int val = 1;
   char valc = 1;
void addrspace(int  __attribute__((address_space(256))) * P) {
   // CHECK: cmpxchg ptr addrspace(256){{.*}}, i32 0, i32 1 seq_cst seq_cst, align 4
 
   __sync_xor_and_fetch(P, 123);
   // CHECK: atomicrmw xor ptr addrspace(256){{.*}}, i32 123 seq_cst, align 4
 }
 
 // Ensure that global initialization of atomics is correct.
 static _Atomic(int *) glob_pointer = (void *)0;
+static _Atomic(int *) glob_pointer_from_int = 0;
+_Atomic(int *) nonstatic_glob_pointer_from_int = 0LL;
 static _Atomic int glob_int = 0;
 static _Atomic float glob_flt = 0.0f;
 
 void force_global_uses(void) {
   (void)glob_pointer;
   // CHECK: %[[LOCAL_INT:.+]] = load atomic i32, ptr @[[GLOB_POINTER]] seq_cst
   // CHECK-NEXT: inttoptr i32 %[[LOCAL_INT]] to ptr
+  (void)glob_pointer_from_int;
+  // CHECK: %[[LOCAL_INT_2:.+]] = load atomic i32, ptr @[[GLOB_POINTER_FROM_INT]] seq_cst
+  // CHECK-NEXT: inttoptr i32 %[[LOCAL_INT_2]] to ptr
+  (void)nonstatic_glob_pointer_from_int;
+  // CHECK: %[[LOCAL_INT_3:.+]] = load atomic i32, ptr @[[NONSTATIC_GLOB_POINTER_FROM_INT]] seq_cst
+  // CHECK-NEXT: inttoptr i32 %[[LOCAL_INT_3]] to ptr
   (void)glob_int;
   // CHECK: load atomic i32, ptr @[[GLOB_INT]] seq_cst
   (void)glob_flt;
   // CHECK: %[[LOCAL_FLT:.+]] = load atomic i32, ptr @[[GLOB_FLT]] seq_cst
   // CHECK-NEXT: bitcast i32 %[[LOCAL_FLT]] to float
 }