[cfe-dev] ILP32, -arch i386 and x86_64 Apple systems

[Richard Smith via cfe-dev]

On Tue, Dec 1, 2015 at 10:16 AM, Jeffrey Walton via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Everyone,
>
> (Re-asking on CFE-Dev). We took a bug report on some line assembly
> code (https://github.com/weidai11/cryptopp/issues/72). I have two
> questions at the end on ILP32, Clang and Apple.
>
> The code that took the bug performs the following:
>
>         asm volatile
>         (
>             // save ebx in case -fPIC is being used
> # if BOOL_X32 || BOOL_X64
>             "pushq %%rbx; cpuid; mov %%ebx, %%edi; popq %%rbx"
> # else
>             "push %%ebx; cpuid; mov %%ebx, %%edi; pop %%ebx"
> # endif
>             : "=a" (output[0]), "=D" (output[1]), "=c" (output[2]),
> "=d" (output[3])
>             : "a" (input), "c" (0)
>         );
>
> The BOOL_* defines are mutually exclusive. BOOL_X32 is set when
> __ILP32__ is defined. BOOL_X86 is defined when __i386__ (and friends)
> is defined. BOOL_X64 is defined when __x86_64__ (and friends) is
> defined. We set the defines based on " System V Application Binary
> Interface, AMD64 (With LP64 and ILP32 Programming Models)" [1].
>

Since no-one else has explicitly pointed this out: this is the wrong
document. When you're targeting i386, the contents of the x86_64 psABI
document are irrelevant, since it is not the ABI document for your
platform. When targeting x86-64, we'll define __ILP32__ as specified in
that document (and it's a bug if we deviate from that), but when targeting
i386, we get to define it however we think is best (and it makes a lot of
sense to define it, because i386 is an ILP32 target) since it's a reserved
identifier.

The compiler error is:
>
>     $ make cpu.o
>     clang++ -DNDEBUG -g2 -O2 -arch i386 -fPIC -march=native -pipe -c
> cpu.cpp
>     cpu.cpp:104:4: error: register %rbx is only available in 64-bit mode
>                             "pushq %%rbx; cpuid; mov %%ebx, %%edi; popq
> %%rbx"
>                            ^
>     <inline asm>:1:8: note: instantiated into assembly here
>            pushq %rbx; cpuid; mov %ebx, %edi; popq %rbx
>               ^~~~~
>     cpu.cpp:104:4: error: register %rbx is only available in 64-bit mode
>                            "pushq %%rbx; cpuid; mov %%ebx, %%edi; popq
> %%rbx"
>                             ^
>     <inline asm>:1:42: note: instantiated into assembly here
>            pushq %rbx; cpuid; mov %ebx, %edi; popq %rbx
>                                                     ^~~~
>     2 errors generated.
>
> It appears Clang sets ILP32 related defines when using -arch i386 on
> x86_64:
>
>     $ clang++ -arch i386 -dM -E - < /dev/null | egrep -i "(86|64|ilp)"
>     #define _ILP32 1
>     #define __ILP32__ 1
>     ...
>     #define __i386 1
>     #define __i386__ 1
>     #define i386 1
>
> As far as I know, when using ILP32 on x86_64, we must interact with
> the stack using 64-bit values and registers.
>
> My first question is, is _ILP32_ and _i386_ supposed to be defined
> together? According to the docs I found, its not an expected/valid
> configuration. _ILP32_ and _x86_64_ is an expected/valid
> configuration.
>
> My second question is, how should I work around this? Should I special
> case Clang on Apple and attempt to push a 32-bit value? Or should I do
> something else?
>
> Thanks in advance.
>
> *****
>
> A related bug report was opened at "LLVM Bug 25688 - Clang defines
> __ILP32__ when using -arch i386 on an x86_64 Intel",
> https://llvm.org/bugs/show_bug.cgi?id=25688.
>
> *****
>
> Here are the docs I found, but its not clear to me what document is
> controlling in this case.
>
> The "OS X ABI Function Call Guide" [0] does not discuss it, and often
> defers to the System V guides.
>
> The "System V Application Binary Interface, AMD64 (With LP64 and ILP32
> Programming Models)" [1] states we should see either (1) __ILP32__
> alone; or (2) both __ILP32__ and __x86_64__ (some hand waiving).
>
> The "SYSTEM V APPLICATION BINARY INTERFACE for Intel 386" [2] does not
> discuss it, and does not mention __ILP32__.
>
> [0]
> http://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/LowLevelABI/Mac_OS_X_ABI_Function_Calls.pdf
> [1] http://sites.google.com/site/x32abi/documents/abi.pdf
> [2] http://www.sco.com/developers/devspecs/abi386-4.pdf
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