[clang][SPARC] Treat empty structs as if it's a one-bit type in the CC (#90338)

Make sure that empty structs are treated as if it has a size of one bit
in function parameters and return types so that it occupies a full
argument and/or return register slot.

This fixes crashes and miscompilations when passing and/or returning
empty structs.

Reviewed by: @s-barannikov

Author: koachan

clang/lib/CodeGen/Targets/Sparc.cpp

@@ -263,7 +263,10 @@ SparcV9ABIInfo::classifyType(QualType Ty, unsigned SizeLimit) const {
 
   CoerceBuilder CB(getVMContext(), getDataLayout());
   CB.addStruct(0, StrTy);
-  CB.pad(llvm::alignTo(CB.DL.getTypeSizeInBits(StrTy), 64));
+  // All structs, even empty ones, should take up a register argument slot,
+  // so pin the minimum struct size to one bit.
+  CB.pad(llvm::alignTo(
+      std::max(CB.DL.getTypeSizeInBits(StrTy).getKnownMinValue(), 1UL), 64));
 
   // Try to use the original type for coercion.
   llvm::Type *CoerceTy = CB.isUsableType(StrTy) ? StrTy : CB.getType();

clang/test/CodeGen/sparcv9-abi.c

@@ -21,6 +21,47 @@ char f_int_4(char x) { return x; }
 // CHECK-LABEL: define{{.*}} fp128 @f_ld(fp128 noundef %x)
 long double f_ld(long double x) { return x; }
 
+// Zero-sized structs reserves an argument register slot if passed directly.
+struct empty {};
+struct emptyarr { struct empty a[10]; };
+
+// CHECK-LABEL: define{{.*}} i64 @f_empty(i64 %x.coerce)
+struct empty f_empty(struct empty x) { return x; }
+
+// CHECK-LABEL: define{{.*}} i64 @f_emptyarr(i64 %x.coerce)
+struct empty f_emptyarr(struct emptyarr x) { return x.a[0]; }
+
+// CHECK-LABEL: define{{.*}} i64 @f_emptyvar(i32 noundef zeroext %count, ...)
+long f_emptyvar(unsigned count, ...) {
+    long ret;
+    va_list args;
+    va_start(args, count);
+
+// CHECK: %[[CUR:[^ ]+]] = load ptr, ptr %args
+// CHECK-DAG: %[[NXT:[^ ]+]] = getelementptr inbounds i8, ptr %[[CUR]], i64 8
+// CHECK-DAG: store ptr %[[NXT]], ptr %args
+    va_arg(args, struct empty);
+
+// CHECK: %[[CUR:[^ ]+]] = load ptr, ptr %args
+// CHECK-DAG: %[[NXT:[^ ]+]] = getelementptr inbounds i8, ptr %[[CUR]], i64 8
+// CHECK-DAG: store ptr %[[NXT]], ptr %args
+// CHECK-DAG: load i64, ptr %[[CUR]]
+    ret = va_arg(args, long);
+    va_end(args);
+    return ret;
+}
+
+// If the zero-sized struct is contained in a non-zero-sized struct,
+// though, it doesn't reserve any registers.
+struct emptymixed { struct empty a; long b; };
+struct emptyflex { unsigned count; struct empty data[10]; };
+
+// CHECK-LABEL: define{{.*}} i64 @f_emptymixed(i64 %x.coerce)
+long f_emptymixed(struct emptymixed x) { return x.b; }
+
+// CHECK-LABEL: define{{.*}} i64 @f_emptyflex(i64 %x.coerce, i64 noundef %y)
+long f_emptyflex(struct emptyflex x, long y) { return y; }
+
 // Small structs are passed in registers.
 struct small {
   int *a, *b;

clang/test/CodeGen/sparcv9-class-return.cpp

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -triple sparcv9-unknown-unknown -emit-llvm %s -o - | FileCheck %s
+
+class Empty {
+};
+
+class Long : public Empty {
+public:
+  long l;
+};
+
+// CHECK: define{{.*}} i64 @_Z4foo15Empty(i64 %e.coerce)
+Empty foo1(Empty e) {
+  return e;
+}
+
+// CHECK: define{{.*}} %class.Long @_Z4foo24Long(i64 %l.coerce)
+Long foo2(Long l) {
+  return l;
+}
+
+// CHECK: define{{.*}} i64 @_Z4foo34Long(i64 %l.coerce)
+long foo3(Long l) {
+  return l.l;
+}