[llvm-dev] Question about instruction selection

Josh Sharp via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 18 18:03:19 PST 2019

I'm new to LLVM and trying to understand instruction selection. I compiled this llvm IR using llc with the '-debug' flag

define i32 @func() #0 {
  %a = alloca i32, align 4
  %1 = load i32, i32* %a, align 4
  ret i32 %1

The resulting assembly is

ADS -8
LDW $r0, $sp, 0                # 4-byte Folded Reload
MOVR $r0, $r0
LDW $r4, $r0, 0
STSP $r0, 0

I'm trying to understand why r0 is being moved to itself. The debug output indicates this:

ISEL: Starting selection on root node: t1: i32 = FrameIndex<0>
ISEL: Starting pattern match
  Morphed node: t1: i32 = MOVR t1
ISEL: Match complete!

In InstrInfo.td, MOVR has this dag pattern: [(set CPURegs:$ra, CPURegs:$rb)]. I don't understand how node t1's pattern matched MOVR's pattern.

Can someone point me to a location in the code where FrameIndex's pattern is defined?

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190119/8f167ca3/attachment.html>

More information about the llvm-dev mailing list