Fix jmp-shortening on x64 when target within instruction.

* lightening/x86.c (jit_try_shorten): If the address is within the
  last instruction, don't shorten.  If the intstruction is a jump, we
  could elide it entirely in some cases, but we don't know if the user
  captured the PC before calling jit_patch_here.  Better to leave this
  to the user.

Thanks to Helmut Eller for the bug report and test case in
https://gitlab.com/wingo/lightening/-/issues/17.

2 files changed, 28 insertions, 0 deletions

diff --git a/lightening/x86.c b/lightening/x86.c
index 5d75eb012..f8ac4b0b8 100644
--- a/lightening/x86.c
+++ b/lightening/x86.c
@@ -362,11 +362,15 @@ jit_try_shorten(jit_state_t *_jit, jit_reloc_t reloc, jit_pointer_t addr)
 {
   uint8_t *loc = _jit->start + reloc.offset;
   uint8_t *start = loc - reloc.inst_start_offset;
+  uint8_t *end = _jit->pc.uc;
   jit_imm_t i0 = (jit_imm_t)addr;
 
   if (loc == start)
     return;
 
+  if (start < (uint8_t*)addr && (uint8_t*)addr <= end)
+    return;
+
   switch (reloc.kind)
     {
     case JIT_RELOC_ABSOLUTE: {
diff --git a/tests/jmp0.c b/tests/jmp0.c
new file mode 100644
index 000000000..261a399b9
--- /dev/null
+++ b/tests/jmp0.c
@@ -0,0 +1,24 @@
+#include "test.h"
+
+static void
+run_test(jit_state_t *j, uint8_t *arena_base, size_t arena_size)
+{
+  jit_begin(j, arena_base, arena_size);
+  size_t align = jit_enter_jit_abi(j, 0, 0, 0);
+  jit_load_args_1(j, jit_operand_gpr (JIT_OPERAND_ABI_WORD, JIT_R0));
+
+  jit_reloc_t r = jit_jmp(j);
+  jit_patch_here(j, r);
+  jit_leave_jit_abi(j, 0, 0, align);
+  jit_retr(j, JIT_R0);
+
+  jit_word_t (*f)(jit_word_t) = jit_end(j, NULL);
+  ASSERT(f(42) == 42);
+  ASSERT(f(-1) == -1);
+}
+
+int
+main (int argc, char *argv[])
+{
+  return main_helper(argc, argv, run_test);
+}