Can't build any programs on 32-bit ARM

[straight-shoota]

Linking any Crystal program (even an empty one) on an arm architecture fails with the following error:

$ cc .build/crystal.o -o .build/crystal  -rdynamic -L/usr/bin/../lib/crystal src/llvm/ext/llvm_ext.o `"/usr/bin/llvm-config-12" --libs --system-libs --ldflags 2> /dev/null` -lstdc++ -lpcre -lm -lgc -lpthread -levent -lrt -ldl
/usr/bin/ld: .build/crystal.o: in function `__muloti4':
/home/thesseyren/Masaüstü/crystal-arm/crystal/src/crystal/compiler_rt/mul.cr:42: undefined reference to `__multi3'
/usr/bin/ld: .build/crystal.o: in function `//':
/home/thesseyren/Masaüstü/crystal-arm/crystal/src/int.cr:105: undefined reference to `__multi3'
/usr/bin/ld: /home/thesseyren/Masaüstü/crystal-arm/crystal/src/int.cr:105: undefined reference to `__multi3'
/usr/bin/ld: .build/crystal.o: in function `_carrying_mul':
/home/thesseyren/Masaüstü/crystal-arm/crystal/src/crystal/compiler_rt/divmod128.cr:45: undefined reference to `__multi3'
/usr/bin/ld: .build/crystal.o: in function `_carrying_mul_add':
/home/thesseyren/Masaüstü/crystal-arm/crystal/src/crystal/compiler_rt/divmod128.cr:56: undefined reference to `__multi3'
/usr/bin/ld: .build/crystal.o:/home/thesseyren/Masaüstü/crystal-arm/crystal/src/crystal/compiler_rt/divmod128.cr:161: more undefined references to `__multi3' follow
collect2: error: ld returned 1 exit status

The function __multi3 is undefined in compiler-rt on 32-bit ARM.

There should be no implicit 128-bit arithmetics in Crystal program, unless you explicitly use those data types. Even the compiler has not yet support for parsing 128-bit integer literals.

#11206 added __muloti4 etc. which do 128-bit arithmetics and are defined as a fun. So they' always get compiled, even when never called.

A quick temporary workaround is this patch, which disables the troublesome functions on arm. They're currently not necessary unless you actually use 128-bit arithmetics.

--- i/src/crystal/compiler_rt/divmod128.cr
+++ w/src/crystal/compiler_rt/divmod128.cr
@@ -1,4 +1,5 @@
 # This file includes an implementation of (U)Int128 modulo/division operations
+{% skip_file if flag?(:arm) %}

 # :nodoc:
 fun __divti3(a : Int128, b : Int128) : Int128
diff --git i/src/crystal/compiler_rt/mul.cr w/src/crystal/compiler_rt/mul.cr
index affe2f2f8..6f90b3346 100644
--- i/src/crystal/compiler_rt/mul.cr
+++ w/src/crystal/compiler_rt/mul.cr
@@ -39,4 +39,3 @@ end

 __mul_impl(__mulosi4, Int32, 32)
 __mul_impl(__mulodi4, Int64, 64)
-__mul_impl(__muloti4, Int128, 128)
+{% unless flag?(:arm) %}
+  __mul_impl(__muloti4, Int128, 128)
+{% end %}

Of course, this is not a real solution because we want to support 128-bit arithmetics on all platforms. Implementing the compiler-rt functions ourselves is actually a step to enable that for 32-bit architectures. It seems we need to implement __multi3 as well. At least for arm.

This was originally reported in https://forum.crystal-lang.org/t/cant-compile-crystal-1-2-2-on-32-bit-arm-armv7l-device-undefined-reference-to-multi3/4077/7

i386 seems to be unaffected, so apparently __multi3 is available.