[LLVMdev] [3.7.0] Two late issues with cross compilation to mips

[Daniel Sanders]

I believe I've identified the problem with almabench but I haven't found the root cause in the compiler yet.

The problem is that a caller saved register ($f14) is being moved across a call and this call sometimes clobbers the value. As a result, the value of the TWOPI constant used in the fmod() calls isn't always 2*PI.

According to -print-after-all, the pass that moves the instruction is Simple Register Coalescing. The bit I'm stuck on at the moment is that I'm not sure what information is supposed to prevent this move from happening. I thought there was supposed to be an ImplicitDefine on the call instruction for each clobbered register but this doesn't seem to be the case. Am I missing something obvious?
________________________________________
From: Daniel Sanders
Sent: 29 July 2015 11:08
To: Hans Wennborg (hans at chromium.org)
Cc: Simon Atanasyan (simon at atanasyan.com); LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
Subject: [3.7.0] Two late issues with cross compilation to mips

Hi,

Sorry for the late report but I've only just found these issues. Llvm.org isn't working for me at the moment but I'll file tickets once it is.

The issues are:

1.       Almabench has some significant numerical differences and fails the reference check for some configs. I'm investigating this one at the moment but early indications are that it's a similar (but different) problem to the one we had in LLVM 3.6.2.

2.       Read-only exception tables have broken compatibility with the ~2 year old gcc toolchains I was using for release testing cross compilation. This isn't a problem for most test-suite runs since we can just update the assembler but is causing trouble for microMIPS. More recent toolchains lack the microMIPS multilib I was using and migrating to the new one is causing link failures. These failures are related to ELF header bits specifying the SNaN/QNaN encodings to be IEEE754-1985 or IEEE754-2008 compliant. I suspect the –mnan=2008 isn't reaching the assembler.

3.       Clang is incompatible with changes to the mips-mti-linux-gnu sysroot from Imagination's mips-mti-linux-gnu toolchain. Libaries are still multilib'd (albeit with a reduced set) but some of the include paths aren't anymore. It's also no longer correct to include sysroot/include (this path is added by common code) since this skips some function definitions. Instead, we must only include sysroot/usr/include like GCC does. There may be more details but so far the fix doesn't look simple. As far as I can tell, clang's multilib expects includes and libraries to have the same layout (osSuffix() seems to control both). The good news is that it's not a regression since we can use toolchains from before this layout change.

Daniel Sanders
Leading Software Design Engineer, MIPS Processor IP
Imagination Technologies Limited
www.imgtec.com<http://www.imgtec.com/>