[BOLT][RISCV] Handle CIE's produced by GNU as

bolt/lib/Core/BinaryFunction.cpp

@@ -2462,6 +2462,13 @@ struct CFISnapshot {
     case MCCFIInstruction::OpDefCfaRegister:
       CFAReg = Instr.getRegister();
       CFARule = UNKNOWN;
+
+      // This shouldn't happen according to the spec but GNU binutils on RISC-V
+      // emits a DW_CFA_def_cfa_register in CIE's which leaves the offset
+      // unspecified. Both readelf and llvm-dwarfdump interpret the offset as 0
+      // in this case so let's do the same.
+      if (CFAOffset == UNKNOWN)
+        CFAOffset = 0;
       break;
     case MCCFIInstruction::OpDefCfaOffset:
       CFAOffset = Instr.getOffset();

bolt/test/RISCV/Inputs/cie-gnu.yaml

@@ -0,0 +1,109 @@
+## Compiled and stripped-down version of:
+## (riscv64-linux-gnu-gcc -nostdlib -static -Wl,-q cie-gnu.s)
+#     .text
+#     .globl _start
+#     .type _start, @function
+# _start:
+#     .cfi_startproc
+#     beq a0, a1, 1f
+#     ret
+# 1:
+#     .cfi_undefined t0 # Arbitrary cfi command to force a new state
+#     ret
+#     .cfi_endproc
+#     .size _start, .-_start
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_RISCV
+  Flags:           [ EF_RISCV_RVC, EF_RISCV_FLOAT_ABI_DOUBLE ]
+  Entry:           0x10144
+ProgramHeaders:
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+        Type:            R_RISCV_32_PCREL
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+...

bolt/test/RISCV/cie-gnu.test

@@ -0,0 +1,17 @@
+# Test that BOLT can handle CIE's produced by GNU as. On RISC-V, GNU as produces
+# the following initial instruction:
+# DW_CFA_def_cfa_register: r2
+# While I believe it is technically incorrect to use this instruction without
+# first using a DW_CFA_def_cfa (since the offset is unspecified), both readelf
+# and llvm-dwarfdump accept this and implicitly set the offset to 0.
+# In BOLT, this used to trigger an assert, however, since it (correctly)
+# believed the offset was not set. This test checks we can handle this
+# situation.
+
+RUN: yaml2obj -o %t %p/Inputs/cie-gnu.yaml
+RUN: llvm-bolt -o %t.bolt %t --reorder-blocks=reverse
+RUN: llvm-dwarfdump --debug-frame %t.bolt | FileCheck %s
+
+CHECK: 0x400000: CFA=X2
+CHECK: 0x400004: CFA=X2: X5=undefined
+CHECK: 0x400006: CFA=X2