lld incorrectly emits region overflow errors when linker relaxation would prevent an overflow

[korran]

I've written an LLD test-case that fails at top-of-tree:

# REQUIRES: riscv                                                                                           

# RUN: rm -rf %t && split-file %s %t && cd %t                                                               

## Check that relaxation prevents region overflow                                                           

# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+c,+relax a.s -o a.32c.o                                
# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+c,+relax b.s -o b.32c.o                                
# RUN: ld.lld -T lds a.32c.o b.32c.o -o 32c                                                                 
# RUN: llvm-objdump --section-headers 32c | FileCheck %s --check-prefixes=RELAX_SECTIONS                    
# RELAX_SECTIONS:   1 .text         0000000a 00000000 TEXT                                                  

## Check that we still overflow with relaxation disabled                                                    

# RUN: not ld.lld -T lds a.32c.o b.32c.o --no-relax -o /dev/null 2>&1 | FileCheck --check-prefix=ERR0 %s    
# ERR0: ld.lld: error: section '.text' will not fit in region 'ROM'                                         

#--- a.s                                                                                                    
.global _start                                                                                              
_start:                                                                                                     
  # These calls can be relaxed to be much smaller, enough to fit within the                                 
  # tiny ROM region                                                                                         
  call bar                                                                                                  
  call bar                                                                                                  
  call bar                                                                                                  
  call bar                                                                                                  

#--- b.s                                                                                                    
.global bar                                                                                                  
bar:                                                                                                        
  ret                                                                                                       

#--- lds                                                                                                    
MEMORY {                                                                                                    
  ROM (rx) : ORIGIN = 0, LENGTH = 12                                                                        
}                                                                                                           
SECTIONS {                                                                                                  
  .text 0x00000 : { *(.text) } > ROM                                                                        
}

Because of this bug, the call to ld.ldd on line 9 will fail with this error:

ld.lld: error: section '.text' will not fit in region 'ROM': overflowed by 22 bytes

If I increase the length of the ROM region, it succeeds, and the .text section in the output file is only 10 bytes; less than the original region size.

$ tail lds
MEMORY {                                                                                                    
  ROM (rx) : ORIGIN = 0, LENGTH = 40                                                                        
}
$ llvm-objdump --section-headers 32c 
Sections:                                                                                                                                                                                                     
Idx Name          Size     VMA      Type                                                                                                                                                                    
  0               00000000 00000000                                                                                                                                                                             
  1 .text         0000000a 00000000 TEXT                                                                                                                                                                
<snip>

I believe the first call to LinkerScript::assignAddresses() is the one that triggers the error, and it occurs before any relaxation passes have been done. Later passes that happen after relaxation don't trigger any errors, but it's too late.

[korran]

My attempt to fix this issue (with test): https://reviews.llvm.org/D149432

[llvmbot]

@llvm/issue-subscribers-lld-elf

[kupiakos]

@korran It looks like LLVM has moved to using Github PRs for review. Do you want me to rebase your fix and send in a PR, or would you like to? My team is lucky to have not hit this issue yet but this could block uprev for us.