Re: [HACKERS] Minor codegen silliness in ExecInterpExpr()

Andres Freund <andres@anarazel.de> writes:
> On 2017-09-28 16:21:34 -0400, Tom Lane wrote:
>> We could save a pointless register spill
>> and reload if there were a temporary variable in there,

> Makes sense.  Do you want to make it so, or shall I?

I just finished testing a patch, as attached.  On my machine (again,
not latest gcc: 4.4.7 on RHEL6 x86_64), it reduces the code size of
execExprInterp.o by a fraction of a percent, and it seems to offer
a slight benefit in "pgbench -S" performance although I'd not put
much stock in that being reproducible.

> I'd personally be
> somewhat tempted to keep the branches in sync here...

I was tempted that way too, but it doesn't apply cleanly to v10,
because of Peter's recent cleanup of function pointer invocation
style.  I don't think it's really worth worrying about.

            regards, tom lane

diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index 09abd46..68a1f96 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -501,15 +501,17 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
         EEO_CASE(EEOP_INNER_SYSVAR)
         {
             int            attnum = op->d.var.attnum;
+            Datum        d;

             /* these asserts must match defenses in slot_getattr */
             Assert(innerslot->tts_tuple != NULL);
             Assert(innerslot->tts_tuple != &(innerslot->tts_minhdr));
-            /* heap_getsysattr has sufficient defenses against bad attnums */

-            *op->resvalue = heap_getsysattr(innerslot->tts_tuple, attnum,
-                                            innerslot->tts_tupleDescriptor,
-                                            op->resnull);
+            /* heap_getsysattr has sufficient defenses against bad attnums */
+            d = heap_getsysattr(innerslot->tts_tuple, attnum,
+                                innerslot->tts_tupleDescriptor,
+                                op->resnull);
+            *op->resvalue = d;

             EEO_NEXT();
         }
@@ -517,15 +519,17 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
         EEO_CASE(EEOP_OUTER_SYSVAR)
         {
             int            attnum = op->d.var.attnum;
+            Datum        d;

             /* these asserts must match defenses in slot_getattr */
             Assert(outerslot->tts_tuple != NULL);
             Assert(outerslot->tts_tuple != &(outerslot->tts_minhdr));

             /* heap_getsysattr has sufficient defenses against bad attnums */
-            *op->resvalue = heap_getsysattr(outerslot->tts_tuple, attnum,
-                                            outerslot->tts_tupleDescriptor,
-                                            op->resnull);
+            d = heap_getsysattr(outerslot->tts_tuple, attnum,
+                                outerslot->tts_tupleDescriptor,
+                                op->resnull);
+            *op->resvalue = d;

             EEO_NEXT();
         }
@@ -533,15 +537,17 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
         EEO_CASE(EEOP_SCAN_SYSVAR)
         {
             int            attnum = op->d.var.attnum;
+            Datum        d;

             /* these asserts must match defenses in slot_getattr */
             Assert(scanslot->tts_tuple != NULL);
             Assert(scanslot->tts_tuple != &(scanslot->tts_minhdr));
-            /* heap_getsysattr has sufficient defenses against bad attnums */

-            *op->resvalue = heap_getsysattr(scanslot->tts_tuple, attnum,
-                                            scanslot->tts_tupleDescriptor,
-                                            op->resnull);
+            /* heap_getsysattr has sufficient defenses against bad attnums */
+            d = heap_getsysattr(scanslot->tts_tuple, attnum,
+                                scanslot->tts_tupleDescriptor,
+                                op->resnull);
+            *op->resvalue = d;

             EEO_NEXT();
         }
@@ -641,13 +647,22 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
          * As both STRICT checks and function-usage are noticeable performance
          * wise, and function calls are a very hot-path (they also back
          * operators!), it's worth having so many separate opcodes.
+         *
+         * Note: the reason for using a temporary variable "d", here and in
+         * other places, is that some compilers think "*op->resvalue = f();"
+         * requires them to evaluate op->resvalue into a register before
+         * calling f(), just in case f() is able to modify op->resvalue
+         * somehow.  The extra line of code can save a useless register spill
+         * and reload, on architectures without many registers.
          */
         EEO_CASE(EEOP_FUNCEXPR)
         {
             FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
+            Datum        d;

             fcinfo->isnull = false;
-            *op->resvalue = op->d.func.fn_addr(fcinfo);
+            d = op->d.func.fn_addr(fcinfo);
+            *op->resvalue = d;
             *op->resnull = fcinfo->isnull;

             EEO_NEXT();
@@ -658,6 +673,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
             FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
             bool       *argnull = fcinfo->argnull;
             int            argno;
+            Datum        d;

             /* strict function, so check for NULL args */
             for (argno = 0; argno < op->d.func.nargs; argno++)
@@ -669,7 +685,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
                 }
             }
             fcinfo->isnull = false;
-            *op->resvalue = op->d.func.fn_addr(fcinfo);
+            d = op->d.func.fn_addr(fcinfo);
+            *op->resvalue = d;
             *op->resnull = fcinfo->isnull;

     strictfail:
@@ -680,11 +697,13 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
         {
             FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
             PgStat_FunctionCallUsage fcusage;
+            Datum        d;

             pgstat_init_function_usage(fcinfo, &fcusage);

             fcinfo->isnull = false;
-            *op->resvalue = op->d.func.fn_addr(fcinfo);
+            d = op->d.func.fn_addr(fcinfo);
+            *op->resvalue = d;
             *op->resnull = fcinfo->isnull;

             pgstat_end_function_usage(&fcusage, true);
@@ -698,6 +717,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
             PgStat_FunctionCallUsage fcusage;
             bool       *argnull = fcinfo->argnull;
             int            argno;
+            Datum        d;

             /* strict function, so check for NULL args */
             for (argno = 0; argno < op->d.func.nargs; argno++)
@@ -712,7 +732,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
             pgstat_init_function_usage(fcinfo, &fcusage);

             fcinfo->isnull = false;
-            *op->resvalue = op->d.func.fn_addr(fcinfo);
+            d = op->d.func.fn_addr(fcinfo);
+            *op->resvalue = d;
             *op->resnull = fcinfo->isnull;

             pgstat_end_function_usage(&fcusage, true);
@@ -1113,6 +1134,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
             if (!op->d.iocoerce.finfo_in->fn_strict || str != NULL)
             {
                 FunctionCallInfo fcinfo_in;
+                Datum        d;

                 fcinfo_in = op->d.iocoerce.fcinfo_data_in;
                 fcinfo_in->arg[0] = PointerGetDatum(str);
@@ -1120,7 +1142,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
                 /* second and third arguments are already set up */

                 fcinfo_in->isnull = false;
-                *op->resvalue = FunctionCallInvoke(fcinfo_in);
+                d = FunctionCallInvoke(fcinfo_in);
+                *op->resvalue = d;

                 /* Should get null result if and only if str is NULL */
                 if (str == NULL)
@@ -1268,6 +1291,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
         EEO_CASE(EEOP_ROWCOMPARE_STEP)
         {
             FunctionCallInfo fcinfo = op->d.rowcompare_step.fcinfo_data;
+            Datum        d;

             /* force NULL result if strict fn and NULL input */
             if (op->d.rowcompare_step.finfo->fn_strict &&
@@ -1279,7 +1303,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)

             /* Apply comparison function */
             fcinfo->isnull = false;
-            *op->resvalue = op->d.rowcompare_step.fn_addr(fcinfo);
+            d = op->d.rowcompare_step.fn_addr(fcinfo);
+            *op->resvalue = d;

             /* force NULL result if NULL function result */
             if (fcinfo->isnull)

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