Re: ngircd test suite failure on mips64el

Christoph Biedl <[email protected]> Tue, 22 Aug 2017 20:21:37 +0200

Newsgroups	gmane.linux.debian.ports.mips
Message-ID	<[email protected]>

affects 871514 + ngircd
found 871514 7.2.0-1
thanks

James Cowgill wrote...

> On 22/08/17 17:51, Christoph Biedl wrote:

> > Ultimately, after rebuilding on mips64el using -O0 the test suite
> > passes. Although it's usually premature to assume it: This smells like
> > a compiler bug.
>
> This is very likely #871514 in gcc-7. Unfortunately it is affecting a
> lot of packages in the archive at the moment causing incorrect code to
> be generated.

Ouch, I see a huge binNMU job approaching.

> The bug happens if a "small" variable is spilled to the stack. GCC may
> emit a store of the smaller size to the stack, but then load it back as
> a 64-bit integer. The top bits of the register will then contain some
> garbage on the stack and cause comparisons like in your example to fail
> (the bottom bits will be 0 but the top bits won't be).

Thanks for sharing, this seems to be the case indeed:

| ./src/ngircd/irc-mode.c:725
|                         if(!is_oper && !is_machine && !is_owner && !is_admin) {
|    29aa8:       16c00141        bnez    s6,29fb0 <Channel_Mode+0xd58>
|    29aac:       0257102a        slt     v0,s2,s7
| ./src/ngircd/irc-mode.c:725 (discriminator 1)
!    29ab0:       dfa20000        ld      v0,0(sp)
|    29ab4:       1440013e        bnez    v0,29fb0 <Channel_Mode+0xd58>
|    29ab8:       0257102a        slt     v0,s2,s7
| ./src/ngircd/irc-mode.c:725 (discriminator 2)
!    29abc:       dfa20018        ld      v0,24(sp)
|    29ac0:       144000c5        bnez    v0,29dd8 <Channel_Mode+0xb80>
|    29ac4:       0257102a        slt     v0,s2,s7
| ./src/ngircd/irc-mode.c:725 (discriminator 3)
!    29ac8:       dfa20020        ld      v0,32(sp)
|    29acc:       10400465        beqz    v0,2ac64 <Channel_Mode+0x1a0c>

The type of the four variables is "bool", so it's certainly something
smaller.

    Christoph

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