[llvm-commits] Commit 61847 broke bootstrap

[Duncan Sands]

Hi Evan, llvm-gcc bootstrap is currently broken on x86-64 linux.
I bisected it down to this commit:

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r61847 | evancheng | 2009-01-07 03:08:57 +0100 (Wed, 07 Jan 2009) | 10 lines

The coalescer does not coalesce a virtual register to a physical register if any of the physical register's sub-register live intervals overlaps with the virtual register. This is overly conservative. It prevents a extract_subreg from being coalesced away:

v1024 = EDI  // not killed
      =
      = EDI

One possible solution is for the coalescer to examine the sub-register live intervals in the same manner as the physical register. Another possibility is to examine defs and uses (when needed) of sub-registers. Both solutions are too expensive. For now, look for "shortvirtual intervals" and scan instructions to look for conflict instead.

This is a small win on x86-64. e.g. It shaves 403.gcc by ~80 instructions.

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It breaks during stage 2, i.e. something was miscompiled during stage 1,
which means I don't have a testcase, sorry.

Ciao,

Duncan.