[builtins] __gnu_[u]ldivmod_helper for libgcc compatibility

[kongyi]

Summary

Android has a legacy issue, where bionic libc used to export some symbols from libgcc, and now huge number of apps relies on this behaviour. In order to maintain compatibility, we need to retain these symbols even after moving to compiler-rt builtins.

Implement __gnu_[u]ldivmod_helper just for Android, since these symbols are deprecated everywhere else.

[peter.smith]

Can you update the description to give some context on why these functions are needed in compiler-rt? To the best of my knowledge gnu_uldivmod_helper is no longer in libgcc and gnu_ldivmod is only called from the v6-m (cortex-m0) implementation of aeabi_ldivmod in libgcc/config/arm/bpabi-v6m.S which would imply that these functions would not not get called.

References:

• libgcc https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00909.html (removal of __gnu_uldivmod_helper)
• Original libgcc patch with motivation to remove these https://patchwork.ozlabs.org/patch/324850/

[kongyi]

In D60809#1469928, @peter.smith wrote:

Can you update the description to give some context on why these functions are needed in compiler-rt? To the best of my knowledge gnu_uldivmod_helper is no longer in libgcc and gnu_ldivmod is only called from the v6-m (cortex-m0) implementation of aeabi_ldivmod in libgcc/config/arm/bpabi-v6m.S which would imply that these functions would not not get called.

References:

• libgcc https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00909.html (removal of __gnu_uldivmod_helper)
• Original libgcc patch with motivation to remove these https://patchwork.ozlabs.org/patch/324850/

Please refer to the updated change description.

[kongyi]

We're going to apply a workaround in Bionic instead.

compiler-rt/lib/builtins/divdi3.c

__divdi3(di_int a, di_int b)
     const int bits_in_dword_m1 = (int)(sizeof(di_int) * CHAR_BIT) - 1;
     di_int s_a = a >> bits_in_dword_m1;           /* s_a = a < 0 ? -1 : 0 */
     di_int s_b = b >> bits_in_dword_m1;           /* s_b = b < 0 ? -1 : 0 */
     a = (a ^ s_a) - s_a;                         /* negate if s_a == -1 */
     b = (b ^ s_b) - s_b;                         /* negate if s_b == -1 */
     s_a ^= s_b;                                  /*sign of quotient */
     return (__udivmoddi4(a, b, (du_int*)0) ^ s_a) - s_a;  /* negate if s_a == -1 */
 }
+
+#if defined(__ANDROID__) && defined(__ARM_EABI__)
+// For libgcc compatibility
+COMPILER_RT_ABI di_int
+__gnu_ldivmod_helper(di_int a, di_int b, di_int *remainder) {
+    di_int quotient = __divdi3 (a, b);
+    *remainder = a - b * quotient;
+    return quotient;
+}
+#endif

compiler-rt/lib/builtins/udivdi3.c

 
 /* Returns: a / b */
 
 COMPILER_RT_ABI du_int
 __udivdi3(du_int a, du_int b)
 {
     return __udivmoddi4(a, b, 0);
 }
+
+#if defined(__ANDROID__) && defined(__ARM_EABI__)
+// For libgcc compatibility
+COMPILER_RT_ABI du_int
+__gnu_uldivmod_helper(du_int a, du_int b, du_int *remainder) {
+    di_int quotient = __udivdi3 (a, b);
+    *remainder = a - b * quotient;
+    return quotient;
+}
+#endif