Demonstrate LLVM behavior with LTO and libcalls

demo1.sh

#!/bin/bash
#
# Both the builtins archive and libc.so expose __aeabi_uidiv. Normally, when
# generating a shared library, the linker links __aeabi_uidiv from the builtins
# archive because it comes first on the linker command-line. However, when
# the only reference to __aeabi_uidiv results from LTO code generation, then the
# linker instead prefers the __aeabi_uidiv from libc.so.

set -e

llvm=/x/clang14

cc="$llvm/bin/clang -target armv7a-linux-androideabi32"
ar=$llvm/bin/llvm-ar
readelf=$llvm/bin/llvm-readelf

cat >mylibc.c <<EOF
  unsigned int __aeabi_uidiv(unsigned int x, unsigned int y) {
    return 42; // approximation
  }
EOF

$cc mylibc.c -fpic -c
rm -f mybuiltins.a && $ar rcs mybuiltins.a mylibc.o
$cc mylibc.o -nostdlib -fpic -shared -fuse-ld=lld -o libmylibc.so

cat >somelib.c <<EOF
  unsigned long long __aeabi_uidiv(unsigned int x, unsigned int y);
  unsigned long long foo(unsigned int x, unsigned int y) {
    #if defined(IMPLICIT)
    return x / y;
    #elif defined(EXPLICIT)
    return __aeabi_uidiv(x, y);
    #else
    #error must define IMPLICIT or EXPLICIT
    #endif
  }
EOF

for src in EXPLICIT IMPLICIT; do
  for lto in "" -flto -flto=thin; do
    echo "### Compile flags: -D$src $lto"
    $cc -O2 -nostdlib -fuse-ld=lld somelib.c -fpic -shared -o libsomelib.so mybuiltins.a libmylibc.so -D$src $lto
    ($readelf -s libsomelib.so | grep __aeabi_uidiv) || true
    echo
  done
done

# OUTPUT
#
# ### Compile flags: -DIMPLICIT 
#      2: 000011d0    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
#     11: 000011d0    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DIMPLICIT -flto
#      1: 00000000     0 FUNC    GLOBAL DEFAULT   UND __aeabi_uidiv
#      9: 00000000     0 FUNC    GLOBAL DEFAULT   UND __aeabi_uidiv
# 
# ### Compile flags: -DIMPLICIT -flto=thin
#      1: 00000000     0 FUNC    GLOBAL DEFAULT   UND __aeabi_uidiv
#      9: 00000000     0 FUNC    GLOBAL DEFAULT   UND __aeabi_uidiv
# 
# ### Compile flags: -DEXPLICIT 
#      2: 000011c0    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
#     11: 000011c0    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DEXPLICIT -flto
#      2: 000011bc    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
#     11: 000011bc    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DEXPLICIT -flto=thin
#      2: 000011bc    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
#     11: 000011bc    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv

demo2.sh

#!/bin/bash
#
# If the builtins library has LLVM bitcode, then the resulting DSO has an ABS
# 0-sized symbol for __aeabi_uidiv.
#

set -e

llvm=/x/clang14

cc="$llvm/bin/clang -target armv7a-linux-androideabi32"
ar=$llvm/bin/llvm-ar
readelf=$llvm/bin/llvm-readelf

cat >mylibc.c <<EOF
  unsigned int __aeabi_uidiv(unsigned int x, unsigned int y) {
    return 42; // approximation
  }
EOF

$cc mylibc.c -fpic -c -flto
rm -f mybuiltins.a && $ar rcs mybuiltins.a mylibc.o

cat >somelib.c <<EOF
  unsigned long long __aeabi_uidiv(unsigned int x, unsigned int y);
  unsigned long long foo(unsigned int x, unsigned int y) {
    #if defined(IMPLICIT)
    return x / y;
    #elif defined(EXPLICIT)
    return __aeabi_uidiv(x, y);
    #else
    #error must define IMPLICIT or EXPLICIT
    #endif
  }
EOF

for src in EXPLICIT IMPLICIT; do
  for lto in "" -flto -flto=thin; do
    echo "### Compile flags: -D$src $lto"
    $cc -fuse-ld=lld -nostdlib -O2 somelib.c -fpic -shared -o libsomelib.so mybuiltins.a -D$src $lto
    ($readelf --dyn-syms libsomelib.so | grep __aeabi_uidiv) || true
    echo
  done
done

# OUTPUT
#
# ### Compile flags: -DEXPLICIT 
#      2: 000011b4    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DEXPLICIT -flto
#      2: 000011b4    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DEXPLICIT -flto=thin
#      2: 000011b0    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DIMPLICIT 
#      2: 000011c4    24 FUNC    GLOBAL DEFAULT     6 __aeabi_uidiv
# 
# ### Compile flags: -DIMPLICIT -flto
#      2: 00000000     0 NOTYPE  GLOBAL DEFAULT   ABS __aeabi_uidiv
# 
# ### Compile flags: -DIMPLICIT -flto=thin
#      2: 00000000     0 NOTYPE  GLOBAL DEFAULT   ABS __aeabi_uidiv