[llvm] [ARM] [Windows] Error out on branch relocations that require a symbol offset (PR #101906)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mc

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

This adds the same kind of verification for ARM, as was added for AArch64 in 1e7f592a890aad860605cf5220530b3744e107ba. This allows catching issues at assembly time, instead of having the linker misinterpret the relocations (as the linker ignores the symbol offset). This verifies that the issue fixed by 8dd065d5bc81b0c8ab57f365bb169a5d92928f25 really is fixed, and points out explicitly if the same issue appears elsewhere.

Note that the parameter Value in the adjustFixupValue function is offset by 4 from the value that is stored as immediate in the instructions, so we compare with 4, when we want to make sure that the written immediate will be zero.

---
Full diff: https://github.com/llvm/llvm-project/pull/101906.diff


2 Files Affected:

- (modified) llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp (+24) 
- (modified) llvm/test/MC/ARM/Windows/branch-reloc-offset.s (+38) 


``````````diff
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
index 994b43f1abb49..4e4a19ddf5584 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
@@ -587,6 +587,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
         return 0;
     return 0xffffff & ((Value - 8) >> 2);
   case ARM::fixup_t2_uncondbranch: {
+    if (STI->getTargetTriple().isOSBinFormatCOFF() && !IsResolved &&
+        Value != 4) {
+      // MSVC link.exe and lld do not support this relocation type
+      // with a non-zero offset. ("Value" is offset by 4 at this point.)
+      Ctx.reportError(Fixup.getLoc(),
+                      "cannot perform a PC-relative fixup with a non-zero "
+                      "symbol offset");
+    }
     Value = Value - 4;
     if (!isInt<25>(Value)) {
       Ctx.reportError(Fixup.getLoc(), "Relocation out of range");
@@ -637,6 +645,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
       Ctx.reportError(Fixup.getLoc(), "Relocation out of range");
       return 0;
     }
+    if (STI->getTargetTriple().isOSBinFormatCOFF() && !IsResolved &&
+        Value != 4) {
+      // MSVC link.exe and lld do not support this relocation type
+      // with a non-zero offset. ("Value" is offset by 4 at this point.)
+      Ctx.reportError(Fixup.getLoc(),
+                      "cannot perform a PC-relative fixup with a non-zero "
+                      "symbol offset");
+    }
 
     // The value doesn't encode the low bit (always zero) and is offset by
     // four. The 32-bit immediate value is encoded as
@@ -666,6 +682,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
                          Endian == llvm::endianness::little);
   }
   case ARM::fixup_arm_thumb_blx: {
+    if (STI->getTargetTriple().isOSBinFormatCOFF() && !IsResolved &&
+        Value != 4) {
+      // MSVC link.exe and lld do not support this relocation type
+      // with a non-zero offset. ("Value" is offset by 4 at this point.)
+      Ctx.reportError(Fixup.getLoc(),
+                      "cannot perform a PC-relative fixup with a non-zero "
+                      "symbol offset");
+    }
     // The value doesn't encode the low two bits (always zero) and is offset by
     // four (see fixup_arm_thumb_cp). The 32-bit immediate value is encoded as
     //   imm32 = SignExtend(S:I1:I2:imm10H:imm10L:00)
diff --git a/llvm/test/MC/ARM/Windows/branch-reloc-offset.s b/llvm/test/MC/ARM/Windows/branch-reloc-offset.s
index 2e70a723ccf78..e7d59cdcf6d40 100644
--- a/llvm/test/MC/ARM/Windows/branch-reloc-offset.s
+++ b/llvm/test/MC/ARM/Windows/branch-reloc-offset.s
@@ -1,4 +1,5 @@
 // RUN: llvm-mc -triple thumbv7-windows-gnu -filetype obj %s -o - | llvm-objdump -D -r - | FileCheck %s
+// RUN: not llvm-mc -triple thumbv7-windows-gnu -filetype obj --defsym ERR=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR
 
     .text
 main:
@@ -55,3 +56,40 @@ main:
 // CHECK:        e: bf00          nop
 // CHECK: 00000010 <.Lother_target>:
 // CHECK:       10: 4770          bx      lr
+
+.ifdef ERR
+    .section "other2", "xr"
+err:
+    nop
+
+// Test errors, if referencing a symbol with an offset
+
+    b .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+    bl .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+    blx .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+
+// Test errors, if referencing a private label which lacks .def/.scl/.type/.endef, in another
+// section, without an offset. Such symbols are omitted from the output symbol table, so the
+// relocation can't reference them. Such relocations usually are made towards the base of the
+// section plus an offset, but such an offset is not supported with this relocation.
+
+    b .Lerr_target2
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+
+    .def .Lerr_target
+    .scl 3
+    .type 32
+    .endef
+.Lerr_target:
+    nop
+    nop
+    bx lr
+
+    .section "other3", "xr"
+    nop
+.Lerr_target2:
+    bx lr
+.endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/101906