[llvm] [ARM] [Windows] Use IMAGE_SYM_CLASS_STATIC for private functions (PR #101828)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mc

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

For functions with private linkage, pick
IMAGE_SYM_CLASS_STATIC rather than IMAGE_SYM_CLASS_EXTERNAL; GlobalValue::isInternalLinkage() only checks for
InternalLinkage, while GlobalValue::isLocalLinkage() checks for both InternalLinkage and PrivateLinkage.

This matches what the AArch64 target does.

This activates a preexisting fix for the AArch64 target from 1e7f592a890aad860605cf5220530b3744e107ba, for the ARM target as well.

When a relocation points at a symbol, one usually can convey an offset to the symbol by encoding it as an immediate in the instruction. However, for the ARM and AArch64 branch instructions, the immediate stored in the instruction is ignored by MS link.exe (and lld-link matches this aspect). (It would be simple to extend lld-link to support it - but such object files would be incompatible with MS link.exe.)

This was worked around by 1e7f592a890aad860605cf5220530b3744e107ba by emitting symbols into the object file symbol table, for temporary symbols that otherwise would have been omitted, if they have the class IMAGE_SYM_CLASS_STATIC, in order to avoid needing an offset in the relocated instruction.

This change gives the symbols generated from functions with the IR level "private" linkage the right class, to activate that workaround.

This fixes https://github.com/llvm/llvm-project/issues/100101, fixing code generation for coroutines for Windows on ARM. After the change in f78688134026686288a8d310b493d9327753a022, coroutines generate a function with private linkage, and calls to this function were previously broken for this target.

---
Full diff: https://github.com/llvm/llvm-project/pull/101828.diff


3 Files Affected:

- (modified) llvm/lib/Target/ARM/ARMAsmPrinter.cpp (+3-3) 
- (added) llvm/test/CodeGen/ARM/Windows/private-func.ll (+17) 
- (added) llvm/test/MC/ARM/Windows/branch-reloc-offset.s (+57) 


``````````diff
diff --git a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
index 642739a29d6b0..6cf63055a518b 100644
--- a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -153,9 +153,9 @@ bool ARMAsmPrinter::runOnMachineFunction(MachineFunction &MF) {
     OptimizationGoals = 0;
 
   if (Subtarget->isTargetCOFF()) {
-    bool Internal = F.hasInternalLinkage();
-    COFF::SymbolStorageClass Scl = Internal ? COFF::IMAGE_SYM_CLASS_STATIC
-                                            : COFF::IMAGE_SYM_CLASS_EXTERNAL;
+    bool Local = F.hasLocalLinkage();
+    COFF::SymbolStorageClass Scl = Local ? COFF::IMAGE_SYM_CLASS_STATIC
+                                         : COFF::IMAGE_SYM_CLASS_EXTERNAL;
     int Type = COFF::IMAGE_SYM_DTYPE_FUNCTION << COFF::SCT_COMPLEX_TYPE_SHIFT;
 
     OutStreamer->beginCOFFSymbolDef(CurrentFnSym);
diff --git a/llvm/test/CodeGen/ARM/Windows/private-func.ll b/llvm/test/CodeGen/ARM/Windows/private-func.ll
new file mode 100644
index 0000000000000..2d030ae3fabbb
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/Windows/private-func.ll
@@ -0,0 +1,17 @@
+; RUN: llc -mtriple thumbv7-windows -filetype asm -o - %s | FileCheck %s
+
+define dso_local void @func1() {
+entry:
+  call void @func2()
+  ret void
+}
+
+define private void @func2() {
+entry:
+  ret void
+}
+
+; CHECK:      .def    .Lfunc2;
+; CHECK-NEXT: .scl    3;
+; CHECK-NEXT: .type   32;
+; CHECK-NEXT: .endef
diff --git a/llvm/test/MC/ARM/Windows/branch-reloc-offset.s b/llvm/test/MC/ARM/Windows/branch-reloc-offset.s
new file mode 100644
index 0000000000000..2e70a723ccf78
--- /dev/null
+++ b/llvm/test/MC/ARM/Windows/branch-reloc-offset.s
@@ -0,0 +1,57 @@
+// RUN: llvm-mc -triple thumbv7-windows-gnu -filetype obj %s -o - | llvm-objdump -D -r - | FileCheck %s
+
+    .text
+main:
+    nop
+    b .Ltarget
+    b .Lother_target
+
+// A private label target in the same section
+    .def .Ltarget
+    .scl 3
+    .type 32
+    .endef
+    .p2align 2
+.Ltarget:
+    bx lr
+
+// A private label target in another section
+    .section "other", "xr"
+    nop
+    nop
+    nop
+    nop
+    nop
+    nop
+    nop
+    nop
+    .def .Lother_target
+    .scl 3
+    .type 32
+    .endef
+    .p2align 2
+.Lother_target:
+    bx lr
+
+// Check that both branches have a relocation with a zero offset.
+//
+// CHECK: 00000000 <main>:
+// CHECK:        0: bf00          nop
+// CHECK:        2: f000 b800     b.w     0x6 <main+0x6>          @ imm = #0x0
+// CHECK:                         00000002:  IMAGE_REL_ARM_BRANCH24T      .Ltarget
+// CHECK:        6: f000 b800     b.w     0xa <main+0xa>          @ imm = #0x0
+// CHECK:                         00000006:  IMAGE_REL_ARM_BRANCH24T      .Lother_target
+// CHECK:        a: bf00          nop
+// CHECK: 0000000c <.Ltarget>:
+// CHECK:        c: 4770          bx      lr
+// CHECK: 00000000 <other>:
+// CHECK:        0: bf00          nop
+// CHECK:        2: bf00          nop
+// CHECK:        4: bf00          nop
+// CHECK:        6: bf00          nop
+// CHECK:        8: bf00          nop
+// CHECK:        a: bf00          nop
+// CHECK:        c: bf00          nop
+// CHECK:        e: bf00          nop
+// CHECK: 00000010 <.Lother_target>:
+// CHECK:       10: 4770          bx      lr

``````````

</details>


https://github.com/llvm/llvm-project/pull/101828