https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105740
--- Comment #8 from luoxhu at gcc dot gnu.org --- (In reply to rguent...@suse.de from comment #6) > On Tue, 21 Jun 2022, jakub at gcc dot gnu.org wrote: > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105740 > > > > --- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> --- > > The problem with switch-conversion done multiple times is that when it is > > done > > early, it can do worse job than when it is done late, e.g. we can have > > better > > range information later which allows (unfortunately switch-conversion > > doesn't > > use that yet, there is a PR about it) to ignore some never reachable values > > etc. > > So ideally we either need to be able to undo switch-conversion and redo it > > if > > things have changed, or do it only late and for e.g. inlining costs perform > > it > > only in analysis mode and record somewhere what kind of lowering would be > > done > > and how much it would cost. > > With multiple if-to-switch, don't we risk that we turn some ifs into switch, > > then > > switch-conversion lowers it back to ifs and then another if-to-switch > > matches > > it again and again lowers it? > > Yeah, I think ideally switch conversion would be done as part of switch > lowering (plus maybe an extra if-to-switch). The issue might be what > I said - some passes don't like switches, but they probably need to be > taught. As of inline cost yes, doing likely-switch-converted analysis > would probably work. git diff diff --git a/gcc/passes.def b/gcc/passes.def index b257307e085..1376e7cb28d 100644 --- a/gcc/passes.def +++ b/gcc/passes.def @@ -243,8 +243,6 @@ along with GCC; see the file COPYING3. If not see Clean them up. Failure to do so well can lead to false positives from warnings for erroneous code. */ NEXT_PASS (pass_copy_prop); /* Identify paths that should never be executed in a conforming program and isolate those paths. */ NEXT_PASS (pass_isolate_erroneous_paths); @@ -329,6 +327,7 @@ along with GCC; see the file COPYING3. If not see POP_INSERT_PASSES () NEXT_PASS (pass_simduid_cleanup); NEXT_PASS (pass_lower_vector_ssa); + NEXT_PASS (pass_if_to_switch); NEXT_PASS (pass_lower_switch); NEXT_PASS (pass_cse_reciprocals); NEXT_PASS (pass_reassoc, false /* early_p */); Tried this to add the second if_to_switch before lower_switch, but switch lowering doesn't work same as switch_conversion: ;; Function test2 (test2, funcdef_no=0, decl_uid=1982, cgraph_uid=1, symbol_order=0) beginning to process the following SWITCH statement ((null):0) : ------- switch (_2) <default: <L27> [INV], case 1: <L20> [INV], case 2: <L21> [INV], case 3: <L22> [INV], case 4: <L2 3> [INV], case 5: <L24> [INV], case 6: <L25> [INV]> ;; GIMPLE switch case clusters: JT(values:6 comparisons:6 range:6 density: 100.00%):1-6 Removing basic block 11 ;; basic block 11, loop depth 0 ;; pred: switch (_2) <default: <L27> [INV], case 1: <L20> [INV], case 2: <L21> [INV], case 3: <L22> [INV], case 4: <L2 3> [INV], case 5: <L24> [INV], case 6: <L25> [INV]> ;; succ: 4 ;; 5 ;; 6 ;; 7 ;; 8 ;; 9 ;; 10 Updating SSA: Registering new PHI nodes in block #0 Registering new PHI nodes in block #2 Updating SSA information for statement _1 = f_10(D)->len; Registering new PHI nodes in block #3 Updating SSA information for statement _2 = f_10(D)->arr[3]; ... int test2 (struct fs * f) { int _1; int _2; int _8; <bb 2> [local count: 1073741824]: _1 = f_10(D)->len; if (_1 > 3) goto <bb 3>; [50.00%] else goto <bb 10>; [50.00%] <bb 3> [local count: 536870913]: _2 = f_10(D)->arr[3]; switch (_2) <default: <L27> [0.00%], case 1: <L20> [16.67%], case 2: <L21> [16.67%], case 3: <L22> [16.67%], case 4: <L23> [16.67%], case 5: <L24> [16.67%], case 6: <L25> [16.67%]> <bb 4> [local count: 67108864]: <L20>: goto <bb 10>; [100.00%] <bb 5> [local count: 62914560]: <L21>: goto <bb 10>; [100.00%] <bb 6> [local count: 58982400]: <L22>: goto <bb 10>; [100.00%] <bb 7> [local count: 55296000]: <L23>: goto <bb 10>; [100.00%] <bb 8> [local count: 51840000]: <L24>: goto <bb 10>; [100.00%] <bb 9> [local count: 48600000]: <L25>: <bb 10> [local count: 1073741824]: # _8 = PHI <12(4), 27(5), 38(6), 18(7), 58(8), 68(9), 0(3), 0(2)> <L27>: return _8; } ASM still contains indirect jump table like -fno-switch-conversion: test2: .LFB0: .cfi_startproc xorl %eax, %eax cmpl $3, (%rdi) jle .L1 cmpl $6, 16(%rdi) ja .L3 movl 16(%rdi), %eax jmp *.L5(,%rax,8) .section .rodata .align 8 .align 4 .L5: .quad .L3 .quad .L11 .quad .L9 .quad .L8 .quad .L7 .quad .L6 .quad .L4 .text .p2align 4,,10 .p2align 3 .L11: movl $12, %eax .L1: ret .p2align 4,,10 .p2align 3 .L9: movl $27, %eax ret .p2align 4,,10 .p2align 3 .L8: movl $38, %eax ret .p2align 4,,10 .p2align 3 .L7: movl $18, %eax ret .p2align 4,,10 .p2align 3 .L6: movl $58, %eax ret .p2align 4,,10 .p2align 3 .L4: movl $68, %eax ret .L3: xorl %eax, %eax ret .cfi_endproc .LFE0: .size test2, .-test2 Is this bug of lower_switch or expected? From the code, they have different purpose as switch_conversion turns switch to single if-else while lower_switch expand CLUSTERS as a decision tree.
