Re: shift/extract SHIFT_COUNT_TRUNCATED combine bug

[Jeff Law]

On 04/08/14 14:07, Mike Stump wrote:
Something broke in the compiler to cause combine to incorrectly optimize:
(insn 12 11 13 3 (set (reg:SI 604 [ D.6102 ])
         (lshiftrt:SI (subreg/s/u:SI (reg/v:DI 601 [ x ]) 0)
             (reg:SI 602 [ D.6103 ]))) t.c:47 4436 {lshrsi3}
      (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
         (nil)))
(insn 13 12 14 3 (set (reg:SI 605)
         (and:SI (reg:SI 604 [ D.6102 ])
             (const_int 1 [0x1]))) t.c:47 3658 {andsi3}
      (expr_list:REG_DEAD (reg:SI 604 [ D.6102 ])
         (nil)))
(insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
         (zero_extend:DI (reg:SI 605))) t.c:47 4616 {zero_extendsidi2}
      (expr_list:REG_DEAD (reg:SI 605)
         (nil)))

into:

(insn 11 10 12 3 (set (reg:SI 602 [ D.6103 ])
         (not:SI (subreg:SI (reg:DI 595 [ D.6102 ]) 0))) t.c:47 3732 
{one_cmplsi2}
      (expr_list:REG_DEAD (reg:DI 595 [ D.6102 ])
         (nil)))
(note 12 11 13 3 NOTE_INSN_DELETED)
(note 13 12 14 3 NOTE_INSN_DELETED)
(insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
         (zero_extract:DI (reg/v:DI 601 [ x ])
             (const_int 1 [0x1])
             (reg:SI 602 [ D.6103 ]))) t.c:47 4668 {c2_extzvdi}
      (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
         (nil)))

This shows up in:

   FAIL: gcc.c-torture/execute/builtin-bitops-1.c execution,  -Og -g

for me.

diff --git a/gcc/combine.c b/gcc/combine.c
index 708691f..c1f50ff 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -7245,6 +7245,18 @@ make_extraction (enum machine_mode mode, rtx inner, 
HOST_WIDE_INT pos,
        extraction_mode = insn.field_mode;
      }

+  /* On a SHIFT_COUNT_TRUNCATED machine, we can't promote the mode of
+     the extract to a larger size on a variable extract, as previously
+     the position might have been optimized to change a bit of the
+     index of the starting bit that would have been ignored before,
+     but, with a larger mode, will then not be.  If we wanted to do
+     this, we'd have to mask out those bits or prove that those bits
+     are 0.  */
+  if (SHIFT_COUNT_TRUNCATED
+      && pos_rtx
+      && GET_MODE_BITSIZE (extraction_mode) > GET_MODE_BITSIZE (mode))
+    extraction_mode = mode;
+
    /* Never narrow an object, since that might not be safe.  */

    if (mode != VOIDmode

is sufficient to never widen variable extracts on SHIFT_COUNT_TRUNCATED 
machines.  So, the question is, how did people expect this to work?  I didn’t 
spot what changed recently to cause the bad code-gen.  The optimization of sub 
into not is ok, despite how funny it looks, because is feeds into extract which 
we know by SHIFT_COUNT_TRUNCATED is safe.

Is the patch a reasonable way to fix this?
On a SHIFT_COUNT_TRUNCATED target, I don't think it's ever OK to widen a 
shift, variable or constant.

In the case of a variable shift, we could easily have eliminated the 
masking code before or during combine.  For a constant shift amount we 
could have adjusted the constant (see SHIFT_COUNT_TRUNCATED in cse.c)

I think it's just an oversight and it has simply never bit us before.

jeff