Bug 898998 - Turn BL into BLX when doing thumb call relocations and the target is ARM. r=nfroyd

build/unix/elfhack/elfhack.cpp

@@ -240,9 +240,26 @@ private:
             unsigned int word0 = addend & 0xffff,
                          word1 = addend >> 16;
 
-            if (((word0 & 0xf800) != 0xf000) || ((word1 & 0x9000) != 0x9000))
+            /* Encoding T4 of B.W is 10x1 ; Encoding T1 of BL is 11x1. */
+            unsigned int type = (word1 & 0xd000) >> 12;
+            if (((word0 & 0xf800) != 0xf000) || ((type & 0x9) != 0x9))
                 throw std::runtime_error("R_ARM_THM_JUMP24/R_ARM_THM_CALL relocation only supported for B.W <label> and BL <label>");
 
+            /* When the target address points to ARM code, switch a BL to a
+             * BLX. This however can't be done with a B.W without adding a
+             * trampoline, which is not supported as of now. */
+            if ((addr & 0x1) == 0) {
+                if (type == 0x9)
+                    throw std::runtime_error("R_ARM_THM_JUMP24/R_ARM_THM_CALL relocation only supported for BL <label> when label points to ARM code");
+                /* The address of the target is always relative to a 4-bytes
+                 * aligned address, so if the address of the BL instruction is
+                 * not 4-bytes aligned, adjust for it. */
+                if ((base_addr + offset) & 0x2)
+                    tmp += 2;
+                /* Encoding T2 of BLX is 11x0. */
+                type = 0xc;
+            }
+
             unsigned int s = (word0 & (1 << 10)) >> 10;
             unsigned int j1 = (word1 & (1 << 13)) >> 13;
             unsigned int j2 = (word1 & (1 << 11)) >> 11;
@@ -256,7 +273,7 @@ private:
             j2 = ((tmp & (1 << 22)) >> 22) ^ !s;
 
             return 0xf000 | (s << 10) | ((tmp & (0x3ff << 12)) >> 12) |
-                   ((word1 & 0xd000) << 16) | (j1 << 29) | (j2 << 27) | ((tmp & 0xffe) << 15);
+                   (type << 28) | (j1 << 29) | (j2 << 27) | ((tmp & 0xffe) << 15);
         }
     };
 

build/unix/elfhack/inject.c

@@ -22,7 +22,7 @@ extern __attribute__((visibility("hidden"))) void original_init(int argc, char *
 extern __attribute__((visibility("hidden"))) Elf32_Rel relhack[];
 extern __attribute__((visibility("hidden"))) Elf_Ehdr elf_header;
 
-void init(int argc, char **argv, char **env)
+int init(int argc, char **argv, char **env)
 {
     Elf32_Rel *rel;
     Elf_Addr *ptr, *start;
@@ -35,4 +35,7 @@ void init(int argc, char **argv, char **env)
 #ifndef NOINIT
     original_init(argc, argv, env);
 #endif
+    // Ensure there is no tail-call optimization, avoiding the use of the
+    // B.W instruction in Thumb for the call above.
+    return 0;
 }