[llvm-commits] [PATCH] Mips backend. Emit $gp restore instructions.

[Akira Hatanaka]

This patch prevents instructions using $gp from being placed between a jalr
and the instruction that restores the clobbered $gp (please see the
following example).

The current implementation creates load nodes that restore $gp when call
instructions are lowered in MipsISelLowering::LowerCall. However, it does
not add chains or glues that are needed to make sure load/store nodes that
have $gp as an operand are not scheduled between JALR nodes and nodes that
restore $gp after the call.

This patch runs a pre-regalloc pass that emits $gp restore instructions
instead.

- bitcode
@p = external global i32
...
tail call void (...)* @f1() nounwind
%tmp = load i32* @p, align 4, !tbaa !0

- currently produces following code:
...
lw  $25, %call16(f1)($gp)
jalr  $25                        # calls f1. $gp is clobbered.
nop
lw  $3, 16($sp)              # load $gp from stack.
lw  $2, %got(p)($gp)      # this instr uses a clobbered $gp.
addu  $gp, $zero, $3     # $gp is restored here.
...

- produces following code after patch is applied.
...
lw  $25, %call16(f1)($gp)
lw  $16, %got(p)($gp)  # $gp has correct value here.
jalr  $25                     # calls f1. clobbers $gp.
nop
lw  $gp, 16($sp)          # $gp restored.
...
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