[MS] Zero out ECX in __cpuid in intrin.h

[rnk]

Summary

Some CPUID leafs depend on the value of ECX as well as EAX, but we left
it uninitialized.

Originally reported as https://crbug.com/901547

[craig.topper]

LGTM

[hans]

Thanks for fixing!

clang/lib/Headers/intrin.h

 
 /*----------------------------------------------------------------------------*\
 |* Misc
 \*----------------------------------------------------------------------------*/
 #if defined(__i386__) || defined(__x86_64__)
 static __inline__ void __DEFAULT_FN_ATTRS
 __cpuid(int __info[4], int __level) {
   __asm__ ("cpuid" : "=a"(__info[0]), "=b" (__info[1]), "=c"(__info[2]), "=d"(__info[3])
-                   : "a"(__level));
+                   : "a"(__level), "c"(0));
 }
 static __inline__ void __DEFAULT_FN_ATTRS
 __cpuidex(int __info[4], int __level, int __ecx) {
   __asm__ ("cpuid" : "=a"(__info[0]), "=b" (__info[1]), "=c"(__info[2]), "=d"(__info[3])
                    : "a"(__level), "c"(__ecx));
 }
 static __inline__ unsigned __int64 __cdecl __DEFAULT_FN_ATTRS
 _xgetbv(unsigned int __xcr_no) {

clang/test/CodeGen/ms-intrinsics-cpuid.c

+// RUN: %clang_cc1 -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN:         -triple i686-windows-msvc -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN:         -triple x86_64-windows-msvc -emit-llvm %s -o - | FileCheck %s
+
+// intrin.h needs size_t, but -ffreestanding prevents us from getting it from
+// stddef.h.  Work around it with this typedef.
+typedef __SIZE_TYPE__ size_t;
+
+#include <intrin.h>
+
+void test__cpuid(int *info, int level) {
+  __cpuid(info, level);
+}
+// CHECK-LABEL: define {{.*}} @test__cpuid(i32* %{{.*}}, i32 %{{.*}})
+// CHECK: call { i32, i32, i32, i32 } asm "cpuid",
+// CHECK-SAME:   "={ax},={bx},={cx},={dx},{ax},{cx},~{dirflag},~{fpsr},~{flags}"
+// CHECK-SAME:   (i32 %{{.*}}, i32 0)