[AVR] Do not place functions in .progmem.data

Previously, the AVR backend would put functions in .progmem.data. This
is probably a regression from when functions still lived in address
space 0. With this change, only global constants are placed in
.progmem.data.

This is not complete: avr-gcc additionally respects -fdata-sections for
progmem global constants, which LLVM doesn't yet do. But fixing that is
a bit more complicated (and I believe other backends such as RISC-V
might also have similar issues).

Differential Revision: https://reviews.llvm.org/D78212

Author: aykevl

llvm/lib/Target/AVR/AVRTargetObjectFile.cpp

@@ -30,7 +30,7 @@ AVRTargetObjectFile::SelectSectionForGlobal(const GlobalObject *GO,
                                             const TargetMachine &TM) const {
   // Global values in flash memory are placed in the progmem.data section
   // unless they already have a user assigned section.
-  if (AVR::isProgramMemoryAddress(GO) && !GO->hasSection())
+  if (AVR::isProgramMemoryAddress(GO) && !GO->hasSection() && Kind.isReadOnly())
     return ProgmemDataSection;
 
   // Otherwise, we work the same way as ELF.

llvm/test/CodeGen/AVR/sections.ll

@@ -0,0 +1,31 @@
+; RUN: llc < %s -march=avr | FileCheck --check-prefixes=CHECK,NOSECTIONS %s
+; RUN: llc -function-sections -data-sections < %s -march=avr | FileCheck --check-prefixes=CHECK,SECTIONS %s
+
+; Test that functions (in address space 1) are not considered .progmem data.
+
+; CHECK: .text
+; SECTIONS: .text.somefunc,"ax",@progbits
+; CHECK-LABEL: somefunc:
+define void @somefunc() addrspace(1) {
+  ret void
+}
+
+
+; Test whether global variables are placed in the correct section.
+
+; Note: avr-gcc would place this global in .progmem.data.flash with
+; -fdata-sections. The AVR backend does not yet respect -fdata-sections in this
+; case.
+; CHECK: .section .progmem.data,"a",@progbits
+; CHECK-LABEL: flash:
+@flash = addrspace(1) constant i16 3
+
+; NOSECTIONS: .section .rodata,"a",@progbits
+; SECTIONS:   .section .rodata.ram1,"a",@progbits
+; CHECK-LABEL: ram1:
+@ram1 = constant i16 3
+
+; NOSECTIONS: .data
+; SECTIONS:   .section .data.ram2,"aw",@progbits
+; CHECK-LABEL: ram2:
+@ram2 = global i16 3