diff mbox series

[committed] arm: remove incorrect handling of FP bignums in move_or_literal_pool

Message ID 20240516101459.994247-1-rearnsha@arm.com
State Committed
Headers
Series [committed] arm: remove incorrect handling of FP bignums in move_or_literal_pool |

Checks

Context Check Description
linaro-tcwg-bot/tcwg_binutils_build--master-arm warning Patch is already merged
linaro-tcwg-bot/tcwg_binutils_build--master-aarch64 warning Patch is already merged

Commit Message

Richard Earnshaw May 16, 2024, 10:14 a.m. UTC 
  This hunk of code in move_or_literal_pool just looks wrong, but I
can't find a testcase that will tickle it to prove it.  It looks a bit
like it was intended to catch cases where a bignum contained a
floating-point value, but there were a number of problems with it.

- It tested X_add_number == -1, but an FP bignum is indicated by any
  value <= 0.
- It converted the floating-point value to extended precision, but
  that's not used on Arm beyond the legacy FPA code.  No attempt was
  made to match the FP value to the intended memory/mov operation.

Since I can't construct a viable testcase, I've just removed the
existing code and made the function error out in this case: this seems
more sensible than generating wrong code or trying to write something
more complex that can't be tested anyway.

---
 gas/config/tc-arm.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)
diff mbox series

Patch

diff --git a/gas/config/tc-arm.c b/gas/config/tc-arm.c
index 343b2e77d7c..41bcfb8dee2 100644
--- a/gas/config/tc-arm.c
+++ b/gas/config/tc-arm.c
@@ -8922,14 +8922,32 @@  move_or_literal_pool (int i, enum lit_type t, bool mode_3)
       uint64_t v;
       if (inst.relocs[0].exp.X_op == O_big)
 	{
-	  LITTLENUM_TYPE w[X_PRECISION];
-	  LITTLENUM_TYPE * l;
+	  LITTLENUM_TYPE *l;
 
-	  if (inst.relocs[0].exp.X_add_number == -1)
+	  if (inst.relocs[0].exp.X_add_number <= 0)  /* FP value.  */
 	    {
-	      gen_to_words (w, X_PRECISION, E_PRECISION);
-	      l = w;
-	      /* FIXME: Should we check words w[2..5] ?  */
+	      /* FIXME: The code that was here previously could not
+		 work.  Firstly, it tried to convert a floating point
+		 number into an extended precision format, but only
+		 provided a buffer of 5 littlenums, which was too
+		 small.  Secondly, it then didn't deal with the value
+		 converted correctly, just reading out the first 4
+		 littlenum fields and assuming that could be used
+		 directly.
+
+		 I think the code was intended to handle expressions
+		 such as:
+
+			LDR r0, =1.0
+			VLDR d0, =55.3
+
+		 but the parsers currently don't permit floating-point
+		 literal values to be written this way, so this code
+		 is probably unreachable.  To be safe, we simply
+		 return an error here.  */
+
+	      inst.error = _("constant expression not supported");
+	      return true;
 	    }
 	  else
 	    l = generic_bignum;