[AVR] Fix atomicrmw result value

[aykevl]

Summary

This patch fixes the atomicrmw result value to be the value before the operation instead of the value after the operation. This was a bug, left as a FIXME in the code (see D97127).

From the LangRef:

The contents of memory at the location specified by the ‘<pointer>’ operand are atomically read, modified, and written back. The original value at the location is returned.

Doing this expansion early allows the register allocator to arrange registers in such a way that commutable operations are simply swapped around as needed, which results in shorter code while still being correct.

[benshi001]

llvm/lib/Target/AVR/AVRISelLowering.cpp

1705

Also, do we need to make a helper function int getRegScratch(void) in AVRSubtarget.h, just like I have done as getIORegRAMPZ(void) ? The new helper function will return r16 for avr-tiny devices.

Also, do we need a helper function int getIORegSREG(void) instead of hard coded 0x3f ?

[benshi001]

llvm/lib/Target/AVR/AVRISelLowering.cpp

1733

Do we need a flag RegState::Define for Result ?

[aykevl]

• use getIORegSREG() instead of hardcoding I/O address 0x3f.

[aykevl]

llvm/lib/Target/AVR/AVRISelLowering.cpp

1705

See my comment in D117426.

1733

No, because BuildMI will always set RegState::Define for the last parameter (Result in this case).

[aykevl]

llvm/lib/Target/AVR/AVRISelLowering.cpp

1705

Sorry I mean D117425.

[benshi001]

llvm/test/CodeGen/AVR/atomics/load16.ll

32

I am OK with this patch. Just a bit confusion.

In the next test file load8.ll, RR1 is the address and RD is the loaded value.

ld [[RD:r[0-9]+]], [[RR:(X|Y|Z)]]

I suggest you make them in accordance with each other when committing.

[aykevl]

• small update to the load8.ll test to make it more readable

[aykevl]

llvm/test/CodeGen/AVR/atomics/load16.ll

32

Sounds reasonable. However, I would prefer to do this in a separate patch so that the diff in this patch is smaller. Here it is: D117991
If you accept that patch I can commit them after each other.

[benshi001]

llvm/test/CodeGen/AVR/atomics/load16.ll

32

OK. Please commit them. :)