Correct unpacking of 3 operands on 32-bit archictectures

0a4750f91c83 optimized unpacking by removing a mask operation
when unpacking three packed operands. Unfortunately, that optimization
is only safe on 64-bit architectures.

Here is what happens on 32-bit architectures.

The operands to be packed are 10-bit register numbers that have been
turned into byte offsets:

  aaaaaaaaaa00
  bbbbbbbbbb00
  cccccccccc00

They can be packed into a single word like this:

    30         20         10        0
     |          |          |        |
  aa aaaaaaaabb bbbbbbbbcc cccccccc00

If we call the packed word P, the original operands can be
extracted like this:

  C =  P         band 2#111111111100
  B = (P bsr 10) band 2#111111111100
  A = (P bsr 20) band 2#111111111100

The bug was that A was extracted without the masking:

  A = P bsr 20

That would give A the value:

  aaaaaaaaaaaabb

That would only be safe if the two most significant bits in B
were zeroes.

1 files changed, 2 insertions, 1 deletions

diff --git a/erts/emulator/utils/beam_makeops b/erts/emulator/utils/beam_makeops
index 31cd072d5a..24eace9fc4 100755
--- a/erts/emulator/utils/beam_makeops
+++ b/erts/emulator/utils/beam_makeops
@@ -48,7 +48,7 @@ $pack_shift[4] = ['0', 'BEAM_LOOSE_SHIFT', # Only for 64 bit wordsize
 		  '(3*BEAM_LOOSE_SHIFT)'];
 
 $pack_mask[2]  = ['BEAM_LOOSE_MASK', $WHOLE_WORD];
-$pack_mask[3]  = ['BEAM_TIGHT_MASK', 'BEAM_TIGHT_MASK', $WHOLE_WORD];
+$pack_mask[3]  = ['BEAM_TIGHT_MASK', 'BEAM_TIGHT_MASK', 'BEAM_TIGHT_MASK'];
 $pack_mask[4]  = ['BEAM_LOOSE_MASK', # Only for 64 bit wordsize
 		  'BEAM_LOOSE_MASK',
 		  'BEAM_LOOSE_MASK',
@@ -251,6 +251,7 @@ $args_per_word[5] = 3;
 $args_per_word[6] = 3;
 
 if ($wordsize == 64) {
+    $pack_mask[3] = ['BEAM_TIGHT_MASK', 'BEAM_TIGHT_MASK', $WHOLE_WORD];
     $args_per_word[4] = 4;
 }