binutils: describe gas hello world hack

userland/arch/x86_64/asm_hello.c

@@ -2,15 +2,15 @@
 #include <inttypes.h>
 
 int main(void) {
-	uint32_t in = 1;
-	uint32_t out = 0;
+	uint64_t in = 0xFFFFFFFF;
+	uint64_t out = 0;
 	__asm__ (
-		"mov %[in], %%eax;"
-		"inc %%eax;"
-		"mov %%eax, %[out]"
+		"mov %[in], %%rax;"
+		"inc %%rax;"
+		"movq %%rax, %[out]"
 		: [out] "=g" (out)
 		: [in] "g" (in)
-		: "%eax"
+		: "rax"
 	);
 	assert(out == in + 1);
 }

userland/arch/x86_64/binutils_hack.c

@@ -0,0 +1,18 @@
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+#if 0
+	uint64_t in = 0xFFFFFFFF;
+	uint64_t out = 0;
+	__asm__ (
+		"mov %[in], %%rax;"
+		"myinc %%rax;"
+		"movq %%rax, %[out]"
+		: [out] "=g" (out)
+		: [in] "g" (in)
+		: "rax"
+	);
+	assert(out == in + 1);
+#endif
+}