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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97984] [10/11 Regression] Worse code for -O3
 than -O2 on aarch64 vector multiply-add
Date: Wed, 25 Nov 2020 12:59:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97984

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |11.0
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-11-25
   Target Milestone|---                         |10.3
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We vectorize the add but not the multiplication.  FMA discovery comes after
vectorization so it can't inhibit the transform (and vectorizer costing can=
not
factor that in).  Is there vector madd available?

arm vectorizer costing could honor the fact that there's ldp/stp instructio=
ns
and thus not artifically make a vector load cheaper than two adjacent scalar
loads.  That would only make the costings equal though.

0x3151930 *b_11(D) 1 times unaligned_load (misalign -1) costs 1 in body
0x3151930 _2 + _3 1 times vector_stmt costs 1 in body
0x3151930 <unknown> 1 times vec_construct costs 2 in prologue
0x3151930 _7 1 times unaligned_store (misalign -1) costs 1 in body
0x31569d0 _7 1 times scalar_store costs 1 in body
0x31569d0 _8 1 times scalar_store costs 1 in body
0x31569d0 _2 + _3 1 times scalar_stmt costs 1 in body
0x31569d0 _5 + _6 1 times scalar_stmt costs 1 in body
0x31569d0 *b_11(D) 1 times scalar_load costs 1 in body
0x31569d0 MEM[(long int *)b_11(D) + 8B] 1 times scalar_load costs 1 in body
t.c:5:8: note: Cost model analysis:
  Vector inside of basic block cost: 3
  Vector prologue cost: 2
  Vector epilogue cost: 0
  Scalar cost of basic block: 6
t.c:5:8: note: Basic block will be vectorized using SLP=