[gcc r13-4976] PR target/108229: A minor STV compute_convert_gain tweak on x86.

[Roger Sayle <sayle@gcc.gnu.org>]

https://gcc.gnu.org/g:de59d8bd163a4b2e50ab566441ab49d7212c3356

commit r13-4976-gde59d8bd163a4b2e50ab566441ab49d7212c3356
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Jan 3 13:37:31 2023 +0000

    PR target/108229: A minor STV compute_convert_gain tweak on x86.
    
    This patch addresses PR target/108229, which is a change in code
    generation during the STV pass, due to the recently approved patch
    to handle vec_select (reductions) in the vector unit.  The recent
    change is innocent, but exposes a latent STV "gain" calculation issue
    that is benign (or closely balanced) on most microarchitectures.
    
    The issue is when STV considers converting PLUS with a MEM operand.
    
    On TARGET_64BIT (m=1):
            addq 24(%rdi), %rdx             // 4 bytes
    or with -m32 (m=2)
            addl    24(%esi), %eax          // 3 bytes
            adcl    28(%esi), %edx          // 3 bytes
    is being converted by STV to
            vmovq   24(%rdi), %xmm5         // 5 bytes
            vpaddq  %xmm5, %xmm4, %xmm4     // 4 bytes
    
    The current code in general_scalar_chain::compute_convert_gain
    considers that scalar unit addition is replaced with a vector
    unit addition (usually about the same cost), but doesn't consider
    anything special about MEM operands, assuming that a scalar load
    gains/costs nothing compared to a vector load.  We can allow the
    backend slightly better fine tuning by including in the gain
    calculation that m scalar loads are being replaced by one vector
    load, and when optimizing for size including that we're increasing
    code size (e.g. an extra vmovq instruction for a MEM operand).
    
    This patch is a win on the CSiBE benchmark when compiled with -Os.
    
    2023-01-03  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            PR target/108229
            * config/i386/i386-features.cc
            (general_scalar_chain::compute_convert_gain) <case PLUS>: Consider
            the gain/cost of converting a MEM operand.

Diff:
---
 gcc/config/i386/i386-features.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc
index 0f59be0b6fa..f420f8339e8 100644
--- a/gcc/config/i386/i386-features.cc
+++ b/gcc/config/i386/i386-features.cc
@@ -567,6 +567,14 @@ general_scalar_chain::compute_convert_gain ()
 	      igain -= vector_const_cost (XEXP (src, 0));
 	    if (CONST_INT_P (XEXP (src, 1)))
 	      igain -= vector_const_cost (XEXP (src, 1));
+	    if (MEM_P (XEXP (src, 1)))
+	      {
+		if (optimize_insn_for_size_p ())
+		  igain -= COSTS_N_BYTES (m == 2 ? 3 : 5);
+		else
+		  igain += m * ix86_cost->int_load[2]
+			   - ix86_cost->sse_load[sse_cost_idx];
+	      }
 	    break;
 
 	  case NEG: