From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@postgreSQL.org
Subject: Minor codegen silliness in ExecInterpExpr()
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Date: Thu, 28 Sep 2017 16:21:34 -0400
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I noticed the following while poking around with perf:

       |                             fcinfo->isnull =3D false;
       |b5b:   movb   $0x0,0x1c(%rdx)
       |                             *op->resvalue =3D op->d.func.fn_addr(f=
cinfo);
  0.02 |       mov    0x8(%rbx),%rcx
  1.19 |       mov    %rdx,%rdi
  0.93 |       mov    %rdx,(%rsp)
       |       mov    %rcx,0x8(%rsp)
  0.01 |       callq  *0x28(%rbx)
  2.17 |       mov    0x8(%rsp),%rcx
       |       mov    %rax,(%rcx)
       |                             *op->resnull =3D fcinfo->isnull;
  1.18 |       mov    (%rsp),%rdx
  4.32 |       mov    0x10(%rbx),%rax
  0.06 |       movzbl 0x1c(%rdx),%edx
  9.14 |       mov    %dl,(%rax)

It looks to me like gcc believes it is required to evaluate "op->resvalue"
before invoking the called function, just in case the function somehow has
access to *op and modifies that.  We could save a pointless register spill
and reload if there were a temporary variable in there, ie

		EEO_CASE(EEOP_FUNCEXPR)
		{
			FunctionCallInfo fcinfo =3D op->d.func.fcinfo_data;
+			Datum	fvalue;
=09
			fcinfo->isnull =3D false;
-			*op->resvalue =3D op->d.func.fn_addr(fcinfo);
+			fvalue =3D op->d.func.fn_addr(fcinfo);
+			*op->resvalue =3D fvalue;
			*op->resnull =3D fcinfo->isnull;

			EEO_NEXT();
		}

and likewise in the other FUNCEXPR cases.

This is on a rather old gcc, haven't checked on bleeding-edge versions.

			regards, tom lane


--=20
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