[Hexagon] R_HEX_GD_PLT_B22_PCREL cannot be relaxed.

[sidneym]

Summary

This type of relocation cannot be relaxed. The branch referring to the symbol is redirected to __tls_get_addr.

Without this change lld could error out with ld.lld: error: relocation R_HEX_GD_PLT_B22_PCREL cannot refer to absolute symbol ... in some cases.

[MaskRay]

Calling a STT_TLS symbol (call a@GDPLT) seems strange. Tthe problem here is that a STT_TLS symbol is considered absolute by isAbsoluteValue. It is incorrect to reference a non-preemptible (due to -Bsymbolic) with an R_PC in PIC link.

I know that you had a previous change (D74443) which converted the target symbol to __tls_get_addr (not STT_TLS), which may be somehow related to this hack. Can you shall a bit more context why call a@GDPLT is used while a is STT_TLS?

lld/ELF/Relocations.cpp

1333

This can potentially break other targets.

1337–1340

Move the check here and check config->emachine == EM_HEXAGON

lld/test/ELF/hexagon-tls-gd-symbolic.s

8 ↗

(On Diff #253435)

##

26 ↗

(On Diff #253435)

section index is unneeded.

27 ↗

(On Diff #253435)

Addresses are unneeded. Delete them

[sidneym]

Update and add more context to the testcase.

call a@GDPLT is transformed to call tls_get_addr I added some more context to the testcase to show the sequence. I'm not sure just saying call a@GDPLT is just an alias for call tls_get_addr is 100% correct but that is basically how I think of it. Section 16.10 of the Hexagon ABI discusses this and the link is here:
http://lists.llvm.org/pipermail/llvm-dev/attachments/20190916/21516a52/attachment-0001.pdf

[MaskRay]

OK, I kinda understand the scheme now. Suggest renaming and replacing the test:

% cat /tmp/c/hexagon-tls-gd-nonpreemptible.s
# REQUIRES: hexagon                                                                                                                                   
# RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %s -o %t.o        
# RUN: ld.lld -shared %t.o -o %t.so  
# RUN: llvm-readobj -r %t.so | FileCheck --check-prefix=RELOC %s
# RUN: llvm-objdump -d --no-show-raw-insn --print-imm-hex %t.so | FileCheck %s
                                                                           
## Prior to D77021 lld would error "relocation R_HEX_GD_PLT_B22_PCREL cannot refer to absolute symbol".
## A PC-relative relocation referencing a non-preemptible absolute symbol (due to STT_TLS) is not representable in -pie/-shared mode.
## For this case we will actually patch the symbol to the external __tls_get_addr which is preemptible.
                                                                           
.globl _start                                                              
.type _start, @function                                                    
                                                                           
# RELOC:      Section ({{.*}}) .rela.plt {  
# RELOC-NEXT:   R_HEX_JMP_SLOT - 0x0                                       
# RELOC-NEXT:   R_HEX_JMP_SLOT __tls_get_addr 0x0                  
# RELOC-NEXT: }                    
                                                                           
# CHECK:      { immext(#{{.*}})                                                                                                                       
# CHECK-NEXT:   r2 = add(pc,##{{.*}}) }                                    
# CHECK-NEXT: { immext(#{{.*}})                                                                                                                       
# CHECK-NEXT:   r0 = add(r2,##-{{.*}}) }                                                                                                              
# CHECK-NEXT: { call {{.*}} }                                                                                                                         
# CHECK-NEXT: { r0 = memw(r0+#0x0) } 
                                     
_start:                
  r2 = add(pc,##_GLOBAL_OFFSET_TABLE_@PCREL)
  r0 = add(r2,##a@GDGOT)
  call a@GDPLT
  r0 = memw(r0+#0)

## a is non-preemptible due to STV_HIDDEN visibility.
## We can achieve the same effect with -Bsymbolic.
.section        .tdata,"awT",@progbits
.globl  a
.hidden a
a:
.word 1

and updating the git description accordingly.

[sidneym]

Use Fangrui's updated version of the testcase and rename it to hexagon-tls-gd-nonpreemptible.s. Appreciate the changes and added content it helps clarify the scenario being tested.

[MaskRay]

LGTM. Please change the git description as well.

lld/ELF/Relocations.cpp

1339

We may patch (call a@GDPLT) to (call tls_get_addr), where tls_get_addr is non-preemptible. We can't relax the PLT relocation here.

lld/test/ELF/hexagon-tls-gd-nonpreemptible.s

2

Delete the empty line

lld/ELF/Relocations.cpp

static void scanReloc(InputSectionBase &sec, OffsetGetter &getOffset, RelTy *&i,
   // example, if we are linking a main executable, all dynamic symbols that can
   // be resolved within the executable will actually be resolved that way at
   // runtime, because the main executable is always at the beginning of a search
   // list. We can leverage that fact.
   if (!sym.isPreemptible && (!sym.isGnuIFunc() || config->zIfuncNoplt)) {
     if (expr == R_GOT_PC && !isAbsoluteValue(sym)) {
       expr = target->adjustRelaxExpr(type, relocatedAddr, expr);
     } else {
       // The 0x8000 bit of r_addend of R_PPC_PLTREL24 is used to choose call
       // stub type. It should be ignored if optimized to R_PC.
       if (config->emachine == EM_PPC && expr == R_PPC32_PLTREL)
         addend &= ~0x8000;
+      // R_HEX_GD_PLT_B22_PCREL (call a@GDPLT) is transformed into
+      // call __tls_get_addr even if the symbol is non-preemptible.
+      if (!(config->emachine == EM_HEXAGON && type == R_HEX_GD_PLT_B22_PCREL))
         expr = fromPlt(expr);
     }
   }
 
   // If the relocation does not emit a GOT or GOTPLT entry but its computation
   // uses their addresses, we need GOT or GOTPLT to be created.
   //
   // The 4 types that relative GOTPLT are all x86 and x86-64 specific.
   if (oneof<R_GOTPLTONLY_PC, R_GOTPLTREL, R_GOTPLT, R_TLSGD_GOTPLT>(expr)) {

lld/test/ELF/hexagon-tls-gd-nonpreemptible.s

+
+# REQUIRES: hexagon
+# RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %s -o %t.o
+# RUN: ld.lld -shared %t.o -o %t.so
+# RUN: llvm-readobj -r %t.so | FileCheck --check-prefix=RELOC %s
+# RUN: llvm-objdump -d --no-show-raw-insn --print-imm-hex %t.so | FileCheck %s
+
+## Prior to D77021 lld would error "relocation R_HEX_GD_PLT_B22_PCREL cannot refer to absolute symbol".
+## A PC-relative relocation referencing a non-preemptible absolute symbol (due to STT_TLS) is not representable in -pie/-shared mode.
+## For this case we will actually patch the symbol to the external __tls_get_addr which is preemptible.
+
+.globl _start
+.type _start, @function
+
+# RELOC:      Section ({{.*}}) .rela.plt {
+# RELOC-NEXT:   R_HEX_JMP_SLOT - 0x0
+# RELOC-NEXT:   R_HEX_JMP_SLOT __tls_get_addr 0x0
+# RELOC-NEXT: }
+
+# CHECK:      { immext(#{{.*}})
+# CHECK-NEXT:   r2 = add(pc,##{{.*}}) }
+# CHECK-NEXT: { immext(#{{.*}})
+# CHECK-NEXT:   r0 = add(r2,##-{{.*}}) }
+# CHECK-NEXT: { call {{.*}} }
+# CHECK-NEXT: { r0 = memw(r0+#0x0) }
+
+_start:
+  r2 = add(pc,##_GLOBAL_OFFSET_TABLE_@PCREL)
+  r0 = add(r2,##a@GDGOT)
+  call a@GDPLT
+  r0 = memw(r0+#0)
+
+## a is non-preemptible due to STV_HIDDEN visibility.
+## We can achieve the same effect with -Bsymbolic.
+.section        .tdata,"awT",@progbits
+.globl  a
+.hidden a
+a:
+.word 1