[lld] [lld] Support thumb PLTs (PR #86223)

[Peter Smith via llvm-commits]

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+// REQUIRES: arm
+// RUN: llvm-mc -filetype=obj -arm-add-build-attributes --arch=thumb --mcpu=cortex-m33 %p/Inputs/arm-plt-reloc.s -o %t1
+// RUN: llvm-mc -filetype=obj -arm-add-build-attributes --arch=thumb --mcpu=cortex-m33 %s -o %t2
+// RUN: ld.lld %t1 %t2 -o %t
+// RUN: llvm-objdump --no-print-imm-hex -d %t | FileCheck %s
+// RUN: ld.lld -shared %t1 %t2 -o %t.so
+// RUN: llvm-objdump --no-print-imm-hex -d %t.so | FileCheck --check-prefix=DSO %s
+// RUN: llvm-readobj -S -r %t.so | FileCheck -check-prefix=DSOREL %s
+
+// RUN: llvm-mc -filetype=obj -arm-add-build-attributes --arch=thumbeb --mcpu=cortex-m33 %p/Inputs/arm-plt-reloc.s -o %t1.be
+// RUN: llvm-mc -filetype=obj -arm-add-build-attributes --arch=thumbeb --mcpu=cortex-m33 %s -o %t2.be
+// RUN: ld.lld %t1.be %t2.be -o %t.be
+// RUN: llvm-objdump --no-print-imm-hex -d %t.be | FileCheck %s
+// RUN: ld.lld -shared %t1.be %t2.be -o %t.so.be
+// RUN: llvm-objdump --no-print-imm-hex -d %t.so.be | FileCheck --check-prefix=DSO %s
+// RUN: llvm-readobj -S -r %t.so.be | FileCheck -check-prefix=DSOREL %s
+
+// RUN: ld.lld --be8 %t1.be %t2.be -o %t.be
+// RUN: llvm-objdump --no-print-imm-hex -d %t.be | FileCheck %s
+// RUN: ld.lld --be8 -shared %t1.be %t2.be -o %t.so.be
+// RUN: llvm-objdump --no-print-imm-hex -d %t.so.be | FileCheck --check-prefix=DSO %s
+// RUN: llvm-readobj -S -r %t.so.be | FileCheck -check-prefix=DSOREL %s
+
+// Test PLT entry generation
+ .text
+ .align 2
+ .globl _start
+ .type  _start,%function
+_start:
+// FIXME, interworking is only supported for BL via BLX at the moment, when
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smithp35 wrote:

For a thumb-2 only target there won't be any interworking (no Arm code) so it should be good to test `b.w` and `b<cc>.w` as well, and I don't think you need the FIXME. If the branches aren't working then we've got something else wrong. 

If the comment is for a possible future where Thumb-2 PLT entries can be added when Arm code is added, then perhaps worth making this explicit.

https://github.com/llvm/llvm-project/pull/86223