On Fri, Aug 04, 2017 at 05:41:51PM +0200, Thomas Petazzoni wrote: > > > > The bug can be reproduced by building with the toolchain available at > > > > http://toolchains.free-electrons.com/downloads/2017.08-rc1-fix-binutils/toolchains/mips64r6el-n32/tarballs/mips64r6el-n32--glibc--bleeding-edge-2017.05-1453-ga703fdd-1.tar.bz2 > > > > and building with the attached kernel configuration file. > > > > > > > > It is not clear to me if this is a kernel issue (lack of __multi3 in > > > > arch/mips/lib/), or a gcc bug in that it shouldn't emit a call to this > > > > function. > > > > > > > > FWIW, sparc64 had a similar issue, and they added __multi3 in their > > > > libgcc replacement, see commit > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/sparc/lib?id=1b4af13ff2cc6897557bb0b8d9e2fad4fa4d67aa. > > > > > > I think these days we've given up the stuborn resistance of the old days > > > against adding new libgcc1 functions to the kernel, so we should probably > > > just add it. > > > > > > I'm looking into this but a small wrench into the gear is that I'm still > > > on GCC 6 so I'm off to building myself a cross-gcc first ... > > > > I now can reproduce the issue with vanilla FSF binutils 2.28 and GCC 7.1.0. > > Great! However, looking at the functions that end up calling __multi3, > I'm wondering why suddenly gcc 7.x needs to call such a function, while > the same code was compiling without __multi3 in libgcc with gcc 6.x. Chances are it's something specific to MIPS64 R6. Before trying your config file I also tried a number of other defconfigs and all built well. Here's a test case which generates a reference to __multi3: unsigned long func(unsigned long a, unsigned long b) { return a > (~0UL) / b; } GCC rearanges above statement to: return (unsigned __int128)a * (unsigned __int128) b > 0xffffffff; computing which requires 128 bit intermediate results. This is the code generated for MIPS64 R2: dmultu $4,$5 mfhi $2 sltu $2,$0,$2 And this is for R6: move $2,$4 move $7,$5 # $6/$7 contain the second op move $6,$0 move $4,$0 move $5,$2 # $4/$5 contain the first op sd $31,8($sp) balc __multi3 sltu $2,$0,$2 Ralf
