R_HEX_B15_PCREL_X/R_HEX_B9_PCREL_X can be in shared objects

[sidneym]

Summary

These are sometimes generated by tail-call optimizations and had not been properly handled.

[MaskRay]

I sent a request to download hexagon-application-binary-interface-specification.zip a few days ago but I still don't get a copy...

The only information in the Internet about R_HEX_B15_PCREL_X is probably this patch now.. I'd say this is very inconvenient for anyone who'd like to contribute to verify the correctness. Since there is not any reference available, I can only guess.

What do the _X relocation types represent? Why are some R_PLT_PC while the others are R_PC?

case R_HEX_B9_PCREL:
case R_HEX_B13_PCREL:
case R_HEX_B15_PCREL:
case R_HEX_32_PCREL:
  return R_PC;
case R_HEX_6_PCREL_X:
case R_HEX_B9_PCREL_X:
case R_HEX_B15_PCREL_X:
case R_HEX_B22_PCREL:
case R_HEX_PLT_B22_PCREL:
case R_HEX_B22_PCREL_X:
case R_HEX_B32_PCREL_X:
  return R_PLT_PC;

[bcain]

In D66542#1640502, @MaskRay wrote:

I sent a request to download hexagon-application-binary-interface-specification.zip a few days ago but I still don't get a copy...

The only information in the Internet about R_HEX_B15_PCREL_X is probably this patch now.. I'd say this is very inconvenient for anyone who'd like to contribute to verify the correctness. Since there is not any reference available, I can only guess.

What do the _X relocation types represent? Why are some R_PLT_PC while the others are R_PC?

The _X relocations are for the immediate extenders.

[ruiu]

I generally trust Sid for the Hexagon port of lld, and because this patch seems like fixing thinko in the Hexagon port, I'm fine with this patch. If there's a relocation for Hexagon that doesn't fall into an existing category, we'll need a technical specification (or at least an informal technical document) to accept it, though.

[MaskRay]

R_PLT_PC and R_PC are mutually convertible.

• R_PLT_PC -> R_PC: if the target symbol is non-preemptable, an R_PLT_PC gets optimized to an R_PC, and the R_PC is considered a link-time constant.
• R_PC -> R_PLT_PC: for non-pic code pointing to an external function, an R_PC behaves as an R_PLT_PC, however, it will trigger the creation of a canonical PLT.

Generally, R_PC should be used if the target symbol may be STT_OBJECT, otherwise R_PLT_PC is preferred (because it can prevent a canonical PLT). I don't have a specification now. From the two lists of R_PC and R_PLT_PC, I guess there might be some problems so I asked for a clarification.

[sidneym]

I accidentally moved R_HEX_6_PCREL_X, I only intended to update R_HEX_B15_PCREL_X/R_HEX_B9_PCREL_X.

[sidneym]

Generally, the non-extended branch relocations are never found in objects but one could manually generate them. The rules work like this:

• if (p0) jump external // The assembler picks the relocation, either R_HEX_B15_PCREL or R_HEX_B15_PCREL_X in practice when the label is external it will pick the _X version
• if (p0) jump #external // The programmer says I want the non-extended version, R_HEX_B15_PCREL will be used.
• if (p0) jump ##external // The programmer says I want the extended version, R_HEX_B15_PCREL_X will be used.

Since it is possible for the assembly programmer to demand the non-extended versions of B9/B13/B15 they could also be moved.

[sidneym]

Any comments, ABI is here:
http://lists.llvm.org/pipermail/llvm-dev/2019-September/135177.html

Thanks

[MaskRay]

In D66542#1683283, @sidneym wrote:

Any comments, ABI is here:
http://lists.llvm.org/pipermail/llvm-dev/2019-September/135177.html

Thanks

Thank you for making it public!

The test here is for the -shared case. STB_GLOBAL STV_DEFAULT symbols are preemptable in an object file, so R_PLT_PC relocations to them can not be optimized to R_PC even if the definitions are local.

Do you need a -no-pie or -pie test that checks if the target symbol is a locally defined symbol, it can be optimized to not use the PLT?

Other than the test request (or clarification if this ELF rule is not applicable on Hexagon), this patch looks good to me.

lld/test/ELF/hexagon-shared.s

61–62

Can some of the PLT: checks be changed to PLT-NEXT:? That improves the error messages of FileCheck. It will make layout change easier which can change section addresses. There are several layout changes in the past, e.g. making .gnu.hash enabled by default, reordering readonly sections before .text, allowing PT_LOAD segments to overlap (D66542), etc. Many people are unfamiliar with Hexagon and thus any -NEXT: convenience here will be very useful.

[sidneym]

Updated testcase.

Local jumps are almost always resolved by the assembler unless -mno-fixup is used.

There are no specific rules regarding which relocations are preemptible.

If there are some behaviors you would like me to model on hexagon let me know which tests and I will take a look at them and try to add Hexagon versions.

Thanks,

[MaskRay]

If PLT is needed, you may want to take a look at riscv-plt.s ppc32-abs-pic.s ppc32-call-stub-pic.s. They demonstrate some properties that should be checked: dynamic relocations, PLT instruction sequences, contents of dynamic section, etc.

[sidneym]

Add a new test similar to riscv-plt.s. Update hexagon-shared.s to test -Bsymbolic and .hidden.

lld/ELF/Arch/Hexagon.cpp

RelExpr Hexagon::getRelExpr(RelType type, const Symbol &s,
   case R_HEX_12_X:
   case R_HEX_16_X:
   case R_HEX_32:
   case R_HEX_32_6_X:
   case R_HEX_HI16:
   case R_HEX_LO16:
     return R_ABS;
   case R_HEX_B9_PCREL:
-  case R_HEX_B9_PCREL_X:
   case R_HEX_B13_PCREL:
   case R_HEX_B15_PCREL:
-  case R_HEX_B15_PCREL_X:
   case R_HEX_6_PCREL_X:
   case R_HEX_32_PCREL:
     return R_PC;
+  case R_HEX_B9_PCREL_X:
+  case R_HEX_B15_PCREL_X:
   case R_HEX_B22_PCREL:
   case R_HEX_PLT_B22_PCREL:
   case R_HEX_B22_PCREL_X:
   case R_HEX_B32_PCREL_X:
     return R_PLT_PC;
   case R_HEX_GOTREL_11_X:
   case R_HEX_GOTREL_16_X:
   case R_HEX_GOTREL_32_6_X:

lld/test/ELF/hexagon-plt.s

+# REQUIRES: hexagon
+# RUN: echo '.globl bar, weak; .type bar,@function; .type weak,@function; bar: weak:' > %t1.s
+
+# RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %t1.s -o %t1.o
+# RUN: ld.lld -shared %t1.o -soname=t1.so -o %t1.so
+# RUN: llvm-mc -mno-fixup -filetype=obj -triple=hexagon-unknown-elf %s -o %t.o
+# RUN: ld.lld %t.o %t1.so -z separate-code -o %t
+# RUN: llvm-readelf -S -s %t | FileCheck --check-prefixes=SEC,NM %s
+# RUN: llvm-readobj -r %t | FileCheck --check-prefix=RELOC %s
+# RUN: llvm-readelf -x .got.plt %t | FileCheck --check-prefix=GOTPLT %s
+# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck --check-prefixes=DIS %s
+
+# SEC: .plt PROGBITS {{0*}}00020040
+
+## A canonical PLT has a non-zero st_value. bar and weak are called but their
+## addresses are not taken, so a canonical PLT is not necessary.
+# NM: {{0*}}00000000 0 FUNC GLOBAL DEFAULT UND bar
+# NM: {{0*}}00000000 0 FUNC WEAK   DEFAULT UND weak
+
+## The .got.plt slots relocated by .rela.plt point to .plt
+## This is required by glibc.
+# RELOC:      .rela.plt {
+# RELOC-NEXT:    0x40078 R_HEX_JMP_SLOT bar 0x0
+# RELOC-NEXT:    0x4007C R_HEX_JMP_SLOT weak 0x0
+# RELOC-NEXT: }
+# GOTPLT:      section '.got.plt'
+# GOTPLT-NEXT: 0x00040068 00000000 00000000 00000000 00000000
+# GOTPLT-NEXT: 0x00040078 00000000 00000000
+
+# DIS:      _start:
+## Direct call
+## Call foo directly
+# DIS-NEXT:   { call 0x2003c }
+## Call bar via plt
+# DIS-NEXT:   { call 0x20060 }
+## Call weak via plt
+# DIS-NEXT:   { call 0x20070 }
+# DIS-NEXT: {   immext(#0)
+
+## Call foo directly
+# DIS-NEXT: if (p0) jump:nt 0x2003c }
+# DIS-NEXT: {   immext(#64)
+## Call bar via plt
+# DIS-NEXT: if (p0) jump:nt 0x20060 }
+# DIS-NEXT: {   immext(#64)
+## Call weak via plt
+# DIS-NEXT: if (p0) jump:nt 0x20070 }
+# DIS-NEXT: {   immext(#0)
+
+## Call foo directly
+# DIS-NEXT: r0 = #0 ; jump 0x2003c }
+# DIS-NEXT: {   immext(#0)
+## Call bar via plt
+# DIS-NEXT: r0 = #0 ; jump 0x20060 }
+# DIS-NEXT: {   immext(#0)
+## Call weak via plt
+# DIS-NEXT: r0 = #0 ; jump 0x20070 }
+
+# DIS:      foo:
+# DIS-NEXT:   2003c:
+
+
+# DIS: Disassembly of section .plt:
+
+# DIS: 00020040 .plt:
+# DIS-NEXT:   20040: {  immext(#131072)
+# DIS-NEXT:   20044:    r28 = add(pc,##131112) }
+# DIS-NEXT:   20048: {  r14 -= add(r28,#16)
+# DIS-NEXT:   2004c:    r15 = memw(r28+#8)
+# DIS-NEXT:   20050:    r28 = memw(r28+#4) }
+# DIS-NEXT:   20054: {  r14 = asr(r14,#2)
+# DIS-NEXT:   20058:    jumpr r28 }
+# DIS-NEXT:   2005c: {  trap0(#219) }
+## bar's plt slot
+# DIS-NEXT:   20060: {  immext(#131072)
+# DIS-NEXT:   20064:    r14 = add(pc,##131096) }
+# DIS-NEXT:   20068: {  r28 = memw(r14+#0) }
+# DIS-NEXT:   2006c: {  jumpr r28 }
+## weak's plt slot
+# DIS-NEXT:   20070: {  immext(#131072)
+# DIS-NEXT:   20074:    r14 = add(pc,##131084) }
+# DIS-NEXT:   20078: {  r28 = memw(r14+#0) }
+# DIS-NEXT:   2007c: {  jumpr r28 }
+
+
+.global _start, foo, bar
+.weak weak
+
+_start:
+  call foo
+  call bar
+  call weak
+  if (p0) jump foo
+  if (p0) jump bar
+  if (p0) jump weak
+  { r0 = #0; jump foo }
+  { r0 = #0; jump bar }
+  { r0 = #0; jump weak }
+
+## foo is local and non-preemptale, no PLT is generated.
+foo:
+  jumpr r31

lld/test/ELF/hexagon-shared.s

 # REQUIRES: hexagon
-# RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %s -o %t
+# RUN: llvm-mc -mno-fixup -filetype=obj -triple=hexagon-unknown-elf %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %S/Inputs/hexagon-shared.s -o %t2.o
 # RUN: ld.lld -shared %t2.o -soname so -o %t3.so
-# RUN: ld.lld -shared %t %t3.so -o %t4.so
+# RUN: ld.lld -shared %t.o %t3.so -o %t4.so
+# RUN: ld.lld -Bsymbolic -shared %t.o %t3.so -o %t5.so
 # RUN: llvm-objdump -d -j .plt %t4.so | FileCheck --check-prefix=PLT %s
 # RUN: llvm-objdump -d -j .text %t4.so | FileCheck --check-prefix=TEXT %s
 # RUN: llvm-objdump -D -j .got %t4.so | FileCheck --check-prefix=GOT %s
 # RUN: llvm-readelf -r  %t4.so | FileCheck --check-prefix=RELO %s
+# RUN: llvm-readelf -r  %t5.so | FileCheck --check-prefix=SYMBOLIC %s
 
-.global foo
-foo:
+.global _start, foo, hidden_symbol
+.hidden hidden_symbol
+_start:
+# When -Bsymbolic is specified calls to locally resolvables should
+# not generate a plt.
+call ##foo
+# Calls to hidden_symbols should not trigger a plt.
+call ##hidden_symbol
 
 # _HEX_32_PCREL
 .word _DYNAMIC - .
 call ##bar
 
 # R_HEX_PLT_B22_PCREL
 call bar@PLT
+# R_HEX_B15_PCREL_X
+if (p0) jump bar
+# R_HEX_B9_PCREL_X
+{ r0 = #0; jump bar }
 
 # R_HEX_GOT_11_X and R_HEX_GOT_32_6_X
 r2=add(pc,##_GLOBAL_OFFSET_TABLE_@PCREL)
 r0 = memw (r2+##bar@GOT)
 jumpr r0
 
 # R_HEX_GOT_16_X
 r0 = add(r1,##bar@GOT)
 
+# foo is local so no plt will be generated
+foo:
+  jumpr lr
+
+hidden_symbol:
+  jumpr lr
+
 # R_HEX_32
 .data
 .global var
 .type var,@object
 .p2align 2
 var:
    .word 10
    .size var, 4
 .global pvar
 .type pvar,@object
 pvar:
    .word var
    .size pvar, 4
 
-# PLT: { immext(#131200
-# PLT: r28 = add(pc,##131252) }
-# PLT: { r14 -= add(r28,#16)
-# PLT: r15 = memw(r28+#8)
-# PLT: r28 = memw(r28+#4) }
-# PLT: { r14 = asr(r14,#2)
-# PLT: jumpr r28 }
-# PLT: { trap0(#219) }
-# PLT: immext(#131200)
-# PLT: r14 = add(pc,##131236) }
-# PLT: r28 = memw(r14+#0) }
-# PLT: jumpr r28 }
-
-# TEXT:  10218: 68 00 01 00 00010068
-# TEXT: {   call 0x10270 }
-# TEXT: r0 = add(r1,##-65548) }
+
+# PLT: { immext(#131264
+# PLT-NEXT: r28 = add(pc,##131268) }
+# PLT-NEXT: { r14 -= add(r28,#16)
+# PLT-NEXT: r15 = memw(r28+#8)
+# PLT-NEXT: r28 = memw(r28+#4) }
+# PLT-NEXT: { r14 = asr(r14,#2)
+# PLT-NEXT: jumpr r28 }
+# PLT-NEXT: { trap0(#219) }
+# PLT-NEXT: immext(#131200)
+# PLT-NEXT: r14 = add(pc,##131252) }
+# PLT-NEXT: r28 = memw(r14+#0) }
+# PLT-NEXT: jumpr r28 }
+
+# TEXT:  8c 00 01 00 0001008c
+# TEXT: {   call 0x102d0 }
+# TEXT: if (p0) jump:nt 0x102d0
+# TEXT: r0 = #0 ; jump 0x102d0
+# TEXT: r0 = add(r1,##-65548)
 
 # GOT: .got:
-# GOT: 202f8: 00 00 00 00 00000000 <unknown>
+# GOT:  00 00 00 00 00000000 <unknown>
 
-# RELO: 000202f8  00000121 R_HEX_GLOB_DAT
-# RELO: 00030300  00000406 R_HEX_32
-# RELO: 00030314  00000122 R_HEX_JMP_SLOT
+# RELO: R_HEX_GLOB_DAT
+# RELO: R_HEX_32
+# RELO: Relocation section '.rela.plt' at offset 0x22c contains 2 entries:
+# RELO: R_HEX_JMP_SLOT {{.*}} foo
+# RELO-NEXT: R_HEX_JMP_SLOT {{.*}} bar
+# RELO-NOT: R_HEX_JMP_SLOT {{.*}} hidden
+
+# Make sure that no PLT is generated for a local call.
+# SYMBOLIC: Relocation section '.rela.plt' at offset 0x22c contains 1 entries:
+# SYMBOLIC: R_HEX_JMP_SLOT {{.*}} bar
+# SYMBOLIC-NOT: R_HEX_JMP_SLOT {{.*}} foo