[LLVMdev] Emulate i64 add with 3 instructions

[Villmow, Micah]

you need to use machine instructions and not SDNodes in the pattern.

Here is an example from our backend:
// integer subtraction
// a - b ==> a + (-b)
def SUB_i8 : Pat<(sub GPRI8:$src0, GPRI8:$src1),
    (ADD_i8 GPRI8:$src0, (NEGATE_i8 GPRI8:$src1))>;

Where ADD_i8 and NEGATE_i8 are machine instructions.

Mica
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Johannes Birgmeier
> Sent: Thursday, October 20, 2011 12:21 PM
> To: LLVM Developers Mailing List
> Subject: [LLVMdev] Emulate i64 add with 3 instructions
> 
> Hello!
> 
> Is there a way to tell LLVM how to emulate an instruction with multiple
> others? Specifically, in our processor, there is no instruction for
> adding two i64s; it has to be done like this
> 
> dst_high32:dst_low32 = src1_low32 + src2_low32 (unsigned add; dst_high
> might contain the overflow bit)
> dst_high32 = dst_high32 + src1_high32
> dst_high32 = dst_high32 + src2_high32
> 
> I tried it with patterns like the following in InstrInfo.td (this is
> obviously wrong, but you have to start somewhere):
> 
> def : Pattern<(set LLRegs:$dst, (add LLRegs:$src1, LLRegs:$src2)),
> [(set
> LLRegs:$dst, (anyext (add (trunc LLRegs:$src1), (trunc
> LLRegs:$src2))))]>;
> 
> ... but tablegen complains: "Cannot use 'add' in an output pattern!".
> Is
> this the right way to do it - that is, defining a pattern for an
> operation? ... Or better: What is the right way to do it?
> 
> Any help appreciated!
> 
> Thanks,
> Johannes
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