[crt][LoongArch] Support LoongArch when building without init_array

[tangyouling]

Summary

LoongArch LLVM and GCC are default using init_array, and this logic
is enabled only if CRT_HAS_INITFINI_ARRAY is not defined, so this
change will have no effect by defaul.

Which is controlled by cmake (in crt/CMakeLists.txt). In addition,
other architectures (x86, arm, aarch64, powerpc, riscv and sparc)
do allow such fallback option.

Depends on D136338

[xen0n]

So what's the expected use case then? Are there libc's that are going to benefit from this, or is it just for completeness?

[tangyouling]

In D137002#3893670, @xen0n wrote:

So what's the expected use case then? Are there libc's that are going to benefit from this, or is it just for completeness?

Now there is no actual libc's from which to benefit, just for completeness.

[xry111]

DT_INIT_ARRAY/DT_FINI_ARRAY are enabled by default since https://gcc.gnu.org/r12-1328. I guess Clang has the similar change.

The other architectures have a fallback because they are supported by old GCC and Clang. For us, we don't support old GCC and Clang at all.

And I don't like to provide such a fallback to ancient mechanism in 2022...

[tangyouling]

In D137002#3893792, @xry111 wrote:

DT_INIT_ARRAY/DT_FINI_ARRAY are enabled by default since https://gcc.gnu.org/r12-1328. I guess Clang has the similar change.

The other architectures have a fallback because they are supported by old GCC and Clang. For us, we don't support old GCC and Clang at all.

And I don't like to provide such a fallback to ancient mechanism in 2022...

Yes, let's drop this patch.

[xen0n]

Let's not do this then :)

compiler-rt/lib/crt/crtbegin.c

 
 #ifdef CRT_HAS_INITFINI_ARRAY
 __attribute__((section(".init_array"),
                used)) static void (*__init)(void) = __do_init;
 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .init,\"ax\",@progbits\n\t"
     "call " __USER_LABEL_PREFIX__ "__do_init\n\t"
     ".popsection");
+#elif defined(__loongarch__)
+__asm__(".pushsection .init,\"ax\",@progbits\n\t"
+    "bl " __USER_LABEL_PREFIX__ "__do_init\n\t"
+    ".popsection");
 #elif defined(__riscv)
 __asm__(".pushsection .init,\"ax\",%progbits\n\t"
         "call " __USER_LABEL_PREFIX__ "__do_init\n\t"
         ".popsection");
 #elif defined(__arm__) || defined(__aarch64__)
 __asm__(".pushsection .init,\"ax\",%progbits\n\t"
     "bl " __USER_LABEL_PREFIX__ "__do_init\n\t"
     ".popsection");

 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
     "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
     ".popsection");
 #elif defined(__arm__) || defined(__aarch64__)
 __asm__(".pushsection .fini,\"ax\",%progbits\n\t"
     "bl " __USER_LABEL_PREFIX__ "__do_fini\n\t"
     ".popsection");
+#elif defined(__loongarch__)
+__asm__(".pushsection .fini,\"ax\",@progbits\n\t"
+    "bl " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+    ".popsection");
 #elif defined(__powerpc__) || defined(__powerpc64__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
     "bl " __USER_LABEL_PREFIX__ "__do_fini\n\t"
     "nop\n\t"
     ".popsection");
 #elif defined(__riscv)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
         "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
         ".popsection");
 #elif defined(__sparc__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
     "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
     ".popsection");
 #else
 #error "crtbegin without .init_fini array unimplemented for this architecture"
 #endif  // CRT_HAS_INIT_FINI_ARRAY