GNU bug report logs - #13342
Clang, the FFI, and 8-bit signed integers

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Message #26 received at 13342 <at> debbugs.gnu.org (full text, mbox):

From: ludo <at> gnu.org (Ludovic Courtès)
To: Peter Teeson <peter.teeson <at> me.com>
Cc: 13342 <at> debbugs.gnu.org
Subject: Re: [PARTIALLY SOLVED] bug#13342: Errors trying to build Guile 2.0.7
Date: Sun, 27 Jan 2013 15:11:32 +0100

Hi Peter,

Sorry for the late reply, and thanks for investigating.

I reproduced the bug on GNU/Linux with Clang 3.1.

Peter Teeson <peter.teeson <at> me.com> skribis:

> I think the reason that we get this error is that the "a" argument is declared to be only 8 bits wide
> but is passed in as -1 which is correctly represented as 0xFFFFFFFF
>
> However an 8-bit representation is 0xFF which is only 255.

This is part of the issue.

Consider this example:

--8<---------------cut here---------------start------------->8---
#include <stdint.h>

int64_t
test_sum (int8_t a, int64_t b)
{
  return a + b;
}
--8<---------------cut here---------------end--------------->8---

When compiled with GCC 4.6, the assembly is:

--8<---------------cut here---------------start------------->8---
test_sum:
.LFB0:
	.cfi_startproc
	movsbq	%dil, %rdi
	leaq	(%rdi,%rsi), %rax
	ret
	.cfi_endproc
--8<---------------cut here---------------end--------------->8---

With Clang 3.1, it is:

--8<---------------cut here---------------start------------->8---
test_sum:                               # @test_sum
	.cfi_startproc
# BB#0:
	movslq	%edi, %rax
	addq	%rsi, %rax
	ret
--8<---------------cut here---------------end--------------->8---

The ‘movsbq’ emitted by GCC arranges to keep only the 8 LSBs.  Clang
does no such thing, thus keeping all the bits of the first operand in
the addition.

I looked at Section 3.2.3 (“Parameter Passing”) of the SysV ABI x86_64
PS but couldn’t find any evidence as to what the correct behavior is.

However, on the caller side, both compilers emit the same code.  This
program:

--8<---------------cut here---------------start------------->8---
#include <stdint.h>

extern int64_t test_sum (int8_t a, int64_t b);

int64_t
foo (void)
{
  return test_sum (-1, 123132);
}
--8<---------------cut here---------------end--------------->8---

leads to the following assembly with both compilers:

--8<---------------cut here---------------start------------->8---
foo:                                    # @foo
	.cfi_startproc
	movl	$-1, %edi
	movl	$123132, %esi           # imm = 0x1E0FC
	jmp	test_sum                # TAILCALL
--8<---------------cut here---------------end--------------->8---

(And as we’ve seen, libffi does the same.)

So that seems to indicate that the callee code generated by Clang is
incorrect as it fails to discard the 24 MSBs of its first argument.

Could you report it as a Clang/LLVM bug (keeping 13342 <at> debbugs.gnu.org
Cc’d)?

Thanks!

Ludo’.

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