add ARM buildin setjmp/longjmp support (#3)

* add ARMv7 support

* add ARM buildin setjmp/longjmp support

Author: xzh3836598

md.S

@@ -462,6 +462,40 @@
 #elif defined(__arm__)
 
     /* TODO: FIXME: implement setjmp and longjmp for ARM */
+    .globl _st_md_cxt_save
+        .type _st_md_cxt_save, %function
+        .align 2
+    _st_md_cxt_save:
+        mov     ip, r0 // r0 is the param jmpbuf ptr address.
+        // Save registers like
+        // *ip++ = v1
+        // *ip++ = ...
+        // *ip++ = v6
+        // *ip++ = sl
+        // *ip++ = fp
+        stmia   ip!, {v1-v6, sl, fp} // TODO: compatible with other ARM version.
+        movs    r2, sp
+        stmia   ip!, {r2, lr}
+        mov     r0, #0               // r0 save the return value(0) of setjmp.
+        bx      lr                   // return
+     .size _st_md_cxt_save, .-_st_md_cxt_save
+    
+    .globl _st_md_cxt_restore
+        .type _st_md_cxt_restore, %function
+        .align 2
+    _st_md_cxt_restore:
+        mov     ip, r0 // r0 -> jmp_buf
+        movs    r0, r1 // r1 -> return value
+        // The bellow is a group, that is:
+        // if (r0 == 0) r0 =1;
+        ITT     eq  
+        moveq   r0, #1 // long_jmp should never return 0
+    
+        ldmia   ip!, {v1-v6, sl, fp} // restore registers.
+        ldr     sp, [ip], #4         // restore sp, like: sp=*ip; ip+=4;
+        ldr     lr, [ip], #4
+        bx      lr  
+    .size _st_md_cxt_restore, .-_st_md_cxt_restore
 
 #endif
 

md.h

@@ -421,12 +421,12 @@
         #elif defined(__arm__)
             #define MD_STACK_GROWS_DOWN
 
-            #if defined(__GLIBC__) && __GLIBC__ >= 2
-                /* Merge from https://github.com/michaeltalyansky/state-threads/commit/56554a5c425aee8e7a73782eae23d74d83c4120a */
-                #define MD_GET_SP(_t) (_t)->context[0].__jmpbuf[8]
-            #else
-                #error "ARM/Linux pre-glibc2 not supported yet"
-            #endif /* defined(__GLIBC__) && __GLIBC__ >= 2 */
+            #define MD_USE_BUILTIN_SETJMP
+            
+            #ifndef JB_RSP
+                #define JB_RSP 8 // JB_RSP must be same as the index we save in jmpbuf
+            #endif
+            #define MD_GET_SP(_t) (_t)->context[0].__jmpbuf[JB_RSP]
 
         #elif defined(__s390__)
             #define MD_STACK_GROWS_DOWN