Fwd: [RTL, ARM] subreg of partially invalid register, is it valid RTL?

[Charles Baylis <charles dot baylis at linaro dot org>]

[Retrying in plain text]

Hi

Background info:

This question is prompted from debugging a segfault reported at
https://bugs.launchpad.net/gcc-linaro/+bug/1215925?comments=all. The
segfault only occurs on gcc linaro-4.7 on an arm target[1]. However,
similar RTL appears in fsf 4.7, so it's not a Linaro-specific problem.

The test case is compiled with:
$ gcc -c upsampling_neon.c -march=armv7-a -mfloat-abi=softfp
-mfpu=neon -O3 -frename-registers -mtune=cortex-a9 -marm -g

The question:

The insn which causes the segfault is:
(debug_insn 1548 1547 1886 11 (var_location:V4HI __b (subreg:V4HI
(reg:V8HI 125 d31 [orig:657 vr ] [657]) 0)) upsampling_neon.c:850 -1
     (nil))

The variable vr is declared as a NEON vector of 8 16bit integers, and
-frename-registers has caused it to be separated into 2 64bit parts.
So the register allocation is ok, but if considered in isolation the
part which reads (reg:V8HI 125 d31 [orig:657 vr ] [657]) represents a
non-existent register, as the last physical register is d31 (regnos
125,126).

When this is processed by sched_analyze_2 in sched-deps.c, the subreg
is handled by treating it as the included (reg), and it therefore adds
dependencies on invalid physical registers, which leads to a segfault.
If the variable was not assigned to d31 (as in fsf 4.7), then it would
not crash, but would add false dependencies.

So, my question is: Is the above RTL valid? In other words, should
sched-deps.c handle this, or should all reg RTL expressions represent
fully valid registers?

Thanks for any insight,
Charles

[1] gcc -v reports:
Configured with: /home/cbaylis/srcarea/gcc/gcc-linaro-4.7/configure
--prefix=/home/cbaylis/tools//tools-arm-unknown-linux-gnueabihf-linaro-4.7
--target=arm-unknown-linux-gnueabihf --enable-languages=c,c++
--with-sysroot=/home/cbaylis/tools//sysroot-arm-unknown-linux-gnueabihf-linaro-4.7
--with-arch=armv7-a --with-fpu=vfpv3-d16 --with-float=softfp
--with-thumb