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move contents of PR587 to here.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33333 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -552,3 +552,48 @@ _test6:
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or r3,r3,r0
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blr
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===-------------------------------------------------------------------------===
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Consider a function like this:
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float foo(float X) { return X + 1234.4123f; }
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The FP constant ends up in the constant pool, so we need to get the LR register.
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This ends up producing code like this:
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_foo:
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.LBB_foo_0: ; entry
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mflr r11
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*** stw r11, 8(r1)
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bl "L00000$pb"
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"L00000$pb":
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mflr r2
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addis r2, r2, ha16(.CPI_foo_0-"L00000$pb")
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lfs f0, lo16(.CPI_foo_0-"L00000$pb")(r2)
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fadds f1, f1, f0
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*** lwz r11, 8(r1)
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mtlr r11
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blr
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This is functional, but there is no reason to spill the LR register all the way
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to the stack (the two marked instrs): spilling it to a GPR is quite enough.
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Implementing this will require some codegen improvements. Nate writes:
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"So basically what we need to support the "no stack frame save and restore" is a
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generalization of the LR optimization to "callee-save regs".
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Currently, we have LR marked as a callee-save reg. The register allocator sees
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that it's callee save, and spills it directly to the stack.
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Ideally, something like this would happen:
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LR would be in a separate register class from the GPRs. The class of LR would be
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marked "unspillable". When the register allocator came across an unspillable
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reg, it would ask "what is the best class to copy this into that I *can* spill"
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If it gets a class back, which it will in this case (the gprs), it grabs a free
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register of that class. If it is then later necessary to spill that reg, so be
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it.
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===-------------------------------------------------------------------------===
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