[llvm] [llvm-objdump] Fix output corruption when DWARF reg-name callback misses. (PR #200975)

[James Henderson via llvm-commits]

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+## Regression test for the llvm-objdump LiveVariable::print buffer-and-flush
+## fix. PR /#192353 added an ASCII-packed virtual-register decode fallback to
+## the compact DWARF expression printer. llvm-objdump's GetRegName lambda
+## used to write "<unknown register N>" to the output stream as a side
+## effect before returning empty; when the new fallback then succeeded, the
+## decoded name was appended, producing output like
+## "<unknown register 2454065>%r1".
+##
+## This test exercises the buffer-and-flush fix end-to-end with four
+## variables, each with a different DWARF location, covering the
+## interesting interactions between the lambda's miss and the compact
+## printer's per-op pass:
+##
+##   x: DW_OP_regx ULEB128(0x257231)
+##      Single reg, lambda misses, ASCII fallback rescues -> printer
+##      returns true, buffer discarded, renders "%r1". (Target fix.)
+##
+##   y: DW_OP_regx ULEB128(0x257231), DW_OP_plus
+##      Lambda misses on the regx (ASCII fallback rescues inside the
+##      printer's stack), but DW_OP_plus is not handled by the compact
+##      printer so it bails with "<unknown op DW_OP_plus (34)>" written
+##      directly to OS and returns false. The buffer is then flushed,
+##      producing the rescued reg's miss marker as a trailing false
+##      alarm. This is the multi-op pile-up case: noisy but not the
+##      original corruption.
+##
+##   z: DW_OP_regx ULEB128(100)
+##      Reg num 100 is below the ASCII validator floor, so the fallback
+##      also rejects. Printer returns false from the reg-failure path
+##      with empty OS; the buffer flush surfaces the single
+##      "<unknown register 100>" marker -- same content as pre-fix.
+##
+##   w: DW_OP_regx ULEB128(0x257231), DW_OP_regx ULEB128(0x257232)
+##      Two regs, both ASCII-rescued -> Stack ends with size 2 instead
+##      of 1, printer writes "<stack of size 2, expected 1>" to OS and
+##      returns false. Buffer flush appends both false-alarm reg
+##      markers behind that, producing the multi-marker pile-up the
+##      buffer-and-flush model is least graceful about.
+
+# RUN: llvm-mc -triple armv8a--none-eabi < %s -filetype=obj -o %t.o
+# RUN: llvm-objdump %t.o -d --debug-vars=ascii | FileCheck %s
+
+# CHECK: 00000000 <foo>:
+# CHECK: x = %r1
+# CHECK: y = <unknown op DW_OP_plus (34)><unknown register 2454065>
+# CHECK: z = <unknown register 100>
+# CHECK: w = <stack of size 2, expected 1><unknown register 2454065><unknown register 2454066>
+# CHECK-NOT: <unknown register 2454065>%r1
+
+	.text
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jh7370 wrote:

> As noted, I think you need explanations for how one would recreate this input, in test code itself.

I'm still not seeing any instructions in the test code for how to generate this assembly?

https://github.com/llvm/llvm-project/pull/200975