trying to figure out what's going wrong with this linking

mystery.md

I have an object file created by LLVM:

$ readelf -s test.o

Symbol table '.symtab' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test
     2: 0000000000000010    22 FUNC    LOCAL  DEFAULT    2 start.callMain2
     3: 0000000000000030   213 FUNC    LOCAL  DEFAULT    2 test.main
     4: 0000000000000110    18 FUNC    LOCAL  DEFAULT    2 start.exit2
     5: 0000000000000000     5 FUNC    GLOBAL DEFAULT    2 _start
     6: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __extenddftf2
     7: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __eqtf2

You can see that it makes an external function call to __extenddftf2 as well as __eqtf2 which both need to be resolved by the linker.

Here I have a compiler-rt.o file:

$ readelf -s compiler-rt.o

Symbol table '.symtab' contains 40 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS compiler_rt
     2: 0000000000000860    21 FUNC    LOCAL  DEFAULT    1 math.inf.inf__anon_38
     3: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    3 .LCPI5_0
     4: 0000000000000a40    21 FUNC    LOCAL  DEFAULT    1 math.inf.inf__anon_40
     5: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    4 .LCPI7_0
     6: 0000000000000cb0    18 FUNC    LOCAL  DEFAULT    1 math.inf.inf__anon_42
     7: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    5 .LCPI9_0
     8: 00000000000013f0    34 FUNC    LOCAL  DEFAULT    1 debug.assert
     9: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
    10: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
    11: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
    12: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
    13: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
    14: 0000000000000000   500 FUNC    GLOBAL DEFAULT    1 __extenddftf2
    15: 0000000000000200   427 FUNC    GLOBAL DEFAULT    1 __extendsftf2
    16: 00000000000003b0   329 FUNC    GLOBAL DEFAULT    1 __extendhfsf2
    17: 0000000000000500   450 FUNC    GLOBAL DEFAULT    1 __extendhftf2
    18: 00000000000006d0   393 FUNC    GLOBAL DEFAULT    1 __lesf2
    19: 0000000000000880   440 FUNC    GLOBAL DEFAULT    1 __ledf2
    20: 0000000000000a60   582 FUNC    GLOBAL DEFAULT    1 __letf2
    21: 0000000000000cd0   390 FUNC    GLOBAL DEFAULT    1 __gesf2
    22: 0000000000000e60   437 FUNC    GLOBAL DEFAULT    1 __gedf2
    23: 0000000000001020   579 FUNC    GLOBAL DEFAULT    1 __getf2
    24: 0000000000001270    46 FUNC    GLOBAL DEFAULT    1 __eqsf2
    25: 00000000000012a0    46 FUNC    GLOBAL DEFAULT    1 __eqdf2
    26: 00000000000012d0    46 FUNC    GLOBAL DEFAULT    1 __ltsf2
    27: 0000000000001300    46 FUNC    GLOBAL DEFAULT    1 __ltdf2
    28: 0000000000001330    46 FUNC    GLOBAL DEFAULT    1 __nesf2
    29: 0000000000001360    46 FUNC    GLOBAL DEFAULT    1 __nedf2
    30: 0000000000001390    46 FUNC    GLOBAL DEFAULT    1 __gtsf2
    31: 00000000000013c0    46 FUNC    GLOBAL DEFAULT    1 __gtdf2
    32: 00000000000003b0   329 FUNC    GLOBAL DEFAULT    1 __gnu_h2f_ieee
    33: 00000000000006d0   393 FUNC    GLOBAL DEFAULT    1 __cmpsf2
    34: 0000000000000880   440 FUNC    GLOBAL DEFAULT    1 __cmpdf2
    35: 0000000000000a60   582 FUNC    GLOBAL DEFAULT    1 __cmptf2
    36: 0000000000000a60   582 FUNC    GLOBAL DEFAULT    1 __eqtf2
    37: 0000000000000a60   582 FUNC    GLOBAL DEFAULT    1 __lttf2
    38: 0000000000000a60   582 FUNC    GLOBAL DEFAULT    1 __netf2
    39: 0000000000001020   579 FUNC    GLOBAL DEFAULT    1 __gttf2

You can see it provides both functions. However, after linking:

ld.lld -error-limit=0 -z stack-size=16777216 --gc-sections -m elf_x86_64 -static -o test test.o /home/andy/.cache/zig/o/8fba243ef4ba3750d1cf9c276017e573/libc.a /home/andy/.cache/zig/o/a694ba35d496c525af985782dbb765f3/libcompiler_rt.a

The __cmptf2 function is

$ readelf -s test

Symbol table '.symtab' contains 13 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test
     2: 0000000000201130    22 FUNC    LOCAL  DEFAULT    1 start.callMain2
     3: 0000000000201150   213 FUNC    LOCAL  DEFAULT    1 test.main
     4: 0000000000201230    18 FUNC    LOCAL  DEFAULT    1 start.exit2
     5: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS compiler_rt
     6: 0000000000201120     5 FUNC    GLOBAL DEFAULT    1 _start
     7: 0000000000201250   221 FUNC    GLOBAL DEFAULT    1 __extenddftf2
     8: 0000000000201330     0 FUNC    GLOBAL DEFAULT    1 __eqtf2
     9: 0000000000201330     0 FUNC    GLOBAL DEFAULT    1 __letf2
    10: 0000000000201330     0 FUNC    GLOBAL DEFAULT    1 __cmptf2
    11: 0000000000201330     0 FUNC    GLOBAL DEFAULT    1 __lttf2
    12: 0000000000201330     0 FUNC    GLOBAL DEFAULT    1 __netf2

The binary crashes because the function call to __eqtf2 has been deleted (the Size is 0). However the call to __extenddftf2 has been preserved, as expected.

  2011e2:	e8 49 01 00 00       	call   201330 <__cmptf2>

There is nothing at address 201330. What gives?

The problem went away when I deleted all the aliases of __eqtf2

The ultimate solution to this problem was a mismatch between declared function calling convention and the calling convention used at the callsite. This problem is exacerbated by (1) the fact that LLVM C API for creating a function call does not allow specifying the calling convention; you have to use the C++ API to do so, (2) LLVM IR did not used to have this requirement, and (3) LLVM module verification checks do not check this.