[SOLVED] Building coreboot: undefined reference __udivmoddi4

Issue

In building coreboot I got error regarding during linking:
coreboot/src/console/vtxprintf.c:102: undefined reference to '__udivmoddi4'.

Where can I find the library containing this function?

I'm building coreboot for x86_64 (Lenovo x230) using gcc (8.1.1 20180531).

Coreboot - git hash: f59a052ee8dae6f1378514cb622d229e652ad2f6

Solution

__udivmoddi4 is a function in libgcc which is used to implement a combined unsigned division/modulo operation for what GCC calls DI mode (doubled-up integers, 64-bit on i686). It is used for operations like this one:

unsigned long long
div (unsigned long long a, unsigned long long b, unsigned long long *p)
{
  *p = a % b;
  return a / b;
}

The use of __udivmoddi4 on i386 is a new optimization in GCC 7, related to this patch. Previous versions emitted separate calls to __umoddi3 and __udivdi3, basically doing the same work twice.

Normally, all these functions are provided by libgcc, but Coreboot does not link against the standard libraries. It supplies its own implementations of these functions in payloads/libpayload/libc/64bit_div.c, but __udivmoddi4 has yet to be added there.

Either you implement the function yourself (easiest way would be to call __umoddi3 and __udivdi3 from it), or you use GCC 6 to compile Coreboot for the time being. Lowering the optimization level just for the printf implementation might constitute a workaround.

Answered By - Florian Weimer