[LLVMdev] CodeGen instructions and patterns
grosbach at apple.com
Fri Feb 24 14:35:42 CST 2012
(readding cc llvmdev)
On Feb 24, 2012, at 11:46 AM, محمد ﻋﻤﺮ ﺩﻫﻠﻮﻯ <omerbeg at gmail.com> wrote:
> > otherwise how are the machine instructions being accessed/matched for instruction selection ?
> Have a look at the code in SelectionDAGISel.
> I am looking at the ARM backend.
> In specific, the instruction selection.
> In Select(N), I only see ad-hoc matches on the basis of the opcode in the provided SDNode of the DAG, and then getMachineNode() is being used to generate the machine instruction.
> Am I understanding this correctly ?
That's just the target-specific part. Also look in lib/CodeGen/SelectionDAG.
You may find the debug output from the compiler useful. Try running llc with the "-debug-only=isel" command line option. That will give lots of information about how the selector is going about figuring out which patterns to use.
> Consider the following multiply add insstruction, given in ARMInstrInfo.td
> def MLA : AsMul1I32<0b0000001, (outs GPR:$Rd), (ins GPR:$Rn, GPR:$Rm, GPR:$Ra),
> IIC_iMAC32, "mla", "\t$Rd, $Rn, $Rm, $Ra",
> [(set GPR:$Rd, (add (mul GPR:$Rn, GPR:$Rm), GPR:$Ra))]>
> How do I get this instruction and it corresponding pattern in the selector ?
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