On Sat, 24 Jan 2026 at 19:21, Amit Langote <[email protected]> wrote: > On Sat, Jan 24, 2026 at 5:16 AM Andres Freund <[email protected]> wrote: > > We can use a pg_assume() in table_scan_getnextslot() to make the compiler > > understand. > > Something like this? > > result = sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, > slot); > pg_assume(result == !TupIsNull(slot)); > return result; > > I assume this relies on table_scan_getnextslot() being inlined into > ExecScanExtended()?
I looked at the objdump of this, and it does seem to get rid of the extra check. I did: cd src/backend/executor gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security -Wmissing-variable-declarations -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -O2 -I../../../src/include -D_GNU_SOURCE -g -c nodeSeqscan.c objdump -d -M intel -S nodeSeqscan.o > nodeSeqscan_pg_assume.s Looking at ExecSeqScanWithQual, I see: master: return sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, slot); 22c: 48 8b 07 mov rax,QWORD PTR [rdi] 22f: 4c 89 f2 mov rdx,r14 232: 44 89 fe mov esi,r15d 235: 48 8b 80 40 01 00 00 mov rax,QWORD PTR [rax+0x140] 23c: ff 50 28 call QWORD PTR [rax+0x28] if (table_scan_getnextslot(scandesc, direction, slot)) 23f: 84 c0 test al,al <-- *** this test and the subsequent jump equal are removed 241: 74 6d je 2b0 <ExecSeqScanWithQual+0x100> if (TupIsNull(slot)) 243: 41 f6 46 04 02 test BYTE PTR [r14+0x4],0x2 248: 75 69 jne 2b3 <ExecSeqScanWithQual+0x103> 24a: 48 8b 45 28 mov rax,QWORD PTR [rbp+0x28] And with your pg_assume added: result = sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, slot); 22c: 48 8b 07 mov rax,QWORD PTR [rdi] 22f: 4c 89 f2 mov rdx,r14 232: 44 89 fe mov esi,r15d 235: 48 8b 80 40 01 00 00 mov rax,QWORD PTR [rax+0x140] 23c: ff 50 28 call QWORD PTR [rax+0x28] pg_assume(result == !TupIsNull(slot)); 23f: 41 f6 46 04 02 test BYTE PTR [r14+0x4],0x2 244: 75 62 jne 2a8 <ExecSeqScanWithQual+0xf8> 246: 48 8b 45 28 mov rax,QWORD PTR [rbp+0x28] I didn't test the performance. David
