RFR: 8253757: Add LLVM-based backend for hsdis

[Nick Gasson]

On Wed, 13 Oct 2021 00:00:22 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> This patch expands the newly added system for hsdis backends to include LLVM.
> 
> The actual code in hsdis-llvm.cpp is based heavily on the work by @luhenry, as published in the never integrated PR https://github.com/openjdk/jdk/pull/392. (I have basically just ripped out the binutils-based part of it.)
> 
> Unfortunately I have not been able to make this work properly on Windows. With some additional flags I made it compile without complaints, but it caused hotspot to segfault in `LoadLibrary` (!) in `os::dll_load` when I tried to load the library. This is somewhat ironic, since the initial implementation was created by Ludovic for the very purpose of using it on Windows.
> 
> The lack of Windows support in this patch does not mean it is impossible to get it to work, just that I need to co-operate with someone who has more experience of compiling LLVM on Windows, and/or are more eager to get this combination to work.

The problem is `LLVMDisasmInstruction()` returns zero size as soon as it hits an instruction it doesn't understand. Something kludgy like this works:


diff --git a/src/utils/hsdis/llvm/hsdis-llvm.cpp b/src/utils/hsdis/llvm/hsdis-llvm.cpp
index a491082f14fa..3c50ee8e3b40 100644
--- a/src/utils/hsdis/llvm/hsdis-llvm.cpp
+++ b/src/utils/hsdis/llvm/hsdis-llvm.cpp
@@ -307,6 +307,10 @@ class hsdis_backend : public hsdis_backend_base {
   virtual size_t decode_instruction(uintptr_t p, uintptr_t start, uintptr_t end) {
     char buf[128];
     size_t size = LLVMDisasmInstruction(_dcontext, (uint8_t*)p, (uint64_t)(end - start), (uint64_t)p, buf, sizeof(buf));
+    if (size == 0 && end - start >= 4) {
+      snprintf(buf, sizeof(buf), "\t.word\t#0x%08x", *(uint32_t*)p);
+      size = 4;
+    }
     if (size > 0) {
       (*_printf_callback)(_printf_stream, "%s", buf);
     }



  0x0000ffff94685454:           br      x8
  0x0000ffff94685458:           .word   #0x93e3f4c0
  0x0000ffff9468545c:           udf     #0xffff
[Exception Handler]
  0x0000ffff94685460:           adrp    x8, #-0x795000      ;   {runtime_call handle_exception_from_callee Runtime1 stub}

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PR: https://git.openjdk.java.net/jdk/pull/5920