Reduce ExecSeqScan* code size using pg_assume()

fb9f955025f optimized code generation by using specialized variants of
ExecSeqScan* for [not] having a qual, projection etc. This allowed the
compiler to optimize the code out the code for qual / projection. However, as
observed by David Rowley at the time, the compiler couldn't prove the
opposite, i.e. that the qual etc *are* present.

By using pg_assume(), introduced in d65eb5b1b84, we can tell the compiler that
the relevant variables are non-null.

This reduces the code size to a surprising degree and seems to lead to a small
but reproducible performance gain.

Reviewed-by: Amit Langote <[email protected]> Discussion:
https://postgr.es/m/CA+HiwqFk-MbwhfX_kucxzL8zLmjEt9MMcHi2YF=DyhPrSjsBEA@mail.gmail.com

diff --git a/src/backend/executor/nodeSeqscan.c b/src/backend/executor/nodeSeqscan.c
index ed35c58c2c34616e7fff6d7cc017e1750eae131e..94047d29430d69ec01dd2c2b5daa303eaa7ca09a 100644 (file)
--- a/src/backend/executor/nodeSeqscan.c
+++ b/src/backend/executor/nodeSeqscan.c
@@ -131,8 +131,12 @@ ExecSeqScanWithQual(PlanState *pstate)
 {
    SeqScanState *node = castNode(SeqScanState, pstate);
 
+   /*
+    * Use pg_assume() for != NULL tests to make the compiler realize no
+    * runtime check for the field is needed in ExecScanExtended().
+    */
    Assert(pstate->state->es_epq_active == NULL);
-   Assert(pstate->qual != NULL);
+   pg_assume(pstate->qual != NULL);
    Assert(pstate->ps_ProjInfo == NULL);
 
    return ExecScanExtended(&node->ss,
@@ -153,7 +157,7 @@ ExecSeqScanWithProject(PlanState *pstate)
 
    Assert(pstate->state->es_epq_active == NULL);
    Assert(pstate->qual == NULL);
-   Assert(pstate->ps_ProjInfo != NULL);
+   pg_assume(pstate->ps_ProjInfo != NULL);
 
    return ExecScanExtended(&node->ss,
                            (ExecScanAccessMtd) SeqNext,
@@ -173,8 +177,8 @@ ExecSeqScanWithQualProject(PlanState *pstate)
    SeqScanState *node = castNode(SeqScanState, pstate);
 
    Assert(pstate->state->es_epq_active == NULL);
-   Assert(pstate->qual != NULL);
-   Assert(pstate->ps_ProjInfo != NULL);
+   pg_assume(pstate->qual != NULL);
+   pg_assume(pstate->ps_ProjInfo != NULL);
 
    return ExecScanExtended(&node->ss,
                            (ExecScanAccessMtd) SeqNext,