Original bug ID: 3912
Reporter: anonymous
Assigned to: @xavierleroy
Status: closed (set by @xavierleroy on 2011-06-04T15:59:38Z)
Resolution: won't fix
Priority: normal
Severity: crash
Version: 3.09.0
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @glondu gildor smimram jcristau

Bug description

OCaml native code programs crash when used with recent versions of GNU ld on alpha. This appears to be because ld now implements global optimizations on the gp register (relaxing). The code generator signifies to the linker when this is safe via .prologue commands. However, these appear to be generated incorrectly by OCaml at times. For instance, the code for caml_apply2 is:
caml_apply2:
...
.prologue 0
...
mov $1, $27
.livereg 0x4007e030, 0x00000000
jmp ($1)

As I understand it, a .prologue value of 0 means that the gp register ($27) is unused by the function, however it is clearly used here. Indeed setting the value >0 avoids the segfault (at least until the next function with .prologue incorrectly set to 0). I'm not an alpha assembly expert (most of my knowledge was learned via searching for this bug!), but it looks like the instr_needs_gp function in alpha/emit.mlp needs to at least check for Imov when one operand is $29. This would require some restructuring, so I don't have a patch at the moment, but if you're interested, I can probably come up with one.

This behavior is present in the current binutils package (2.16.1cvs20051117-1) in Debian's unstable distribution. Although this package is based on a CVS snapshot of binutils, the behavior will be included in the next release which shouldn't be too far away. I don't have a small example to reproduce this, buf if you rebuild OCaml on alpha, then ocamlopt.opt will segfault at startup. A temporary workaround is to pass -Wl,--no-relax to gcc which turns off the optimization.

Cheers,
Mike Furr (mfurr at debian.org)