6015a045d7
Let Propeller use specialized IDs for basic blocks, instead of MBB number.
This allows optimizations not just prior to asm-printer, but throughout the entire codegen.
This patch only implements the functionality under the new `LLVM_BB_ADDR_MAP` version, but the old version is still being used. A later patch will change the used version.
####Background
Today Propeller uses machine basic block (MBB) numbers, which already exist, to map native assembly to machine IR. This is done as follows.
- Basic block addresses are captured and dumped into the `LLVM_BB_ADDR_MAP` section just before the AsmPrinter pass which writes out object files. This ensures that we have a mapping that is close to assembly.
- Profiling mapping works by taking a virtual address of an instruction and looking up the `LLVM_BB_ADDR_MAP` section to find the MBB number it corresponds to.
- While this works well today, we need to do better when we scale Propeller to target other Machine IR optimizations like spill code optimization. Register allocation happens earlier in the Machine IR pipeline and we need an annotation mechanism that is valid at that point.
- The current scheme will not work in this scenario because the MBB number of a particular basic block is not fixed and changes over the course of codegen (via renumbering, adding, and removing the basic blocks).
- In other words, the volatile MBB numbers do not provide a one-to-one correspondence throughout the lifetime of Machine IR. Profile annotation using MBB numbers is restricted to a fixed point; only valid at the exact point where it was dumped.
- Further, the object file can only be dumped before AsmPrinter and cannot be dumped at an arbitrary point in the Machine IR pass pipeline. Hence, MBB numbers are not suitable and we need something else.
####Solution
We propose using fixed unique incremental MBB IDs for basic blocks instead of volatile MBB numbers. These IDs are assigned upon the creation of machine basic blocks. We modify `MachineFunction::CreateMachineBasicBlock` to assign the fixed ID to every newly created basic block. It assigns `MachineFunction::NextMBBID` to the MBB ID and then increments it, which ensures having unique IDs.
To ensure correct profile attribution, multiple equivalent compilations must generate the same Propeller IDs. This is guaranteed as long as the MachineFunction passes run in the same order. Since the `NextBBID` variable is scoped to `MachineFunction`, interleaving of codegen for different functions won't cause any inconsistencies.
The new encoding is generated under the new version number 2 and we keep backward-compatibility with older versions.
####Impact on Size of the `LLVM_BB_ADDR_MAP` Section
Emitting the Propeller ID results in a 23% increase in the size of the `LLVM_BB_ADDR_MAP` section for the clang binary.
Reviewed By: tmsriram
Differential Revision: https://reviews.llvm.org/D100808
146 lines
4.9 KiB
YAML
146 lines
4.9 KiB
YAML
# Start after bbsections0-prepare and check that the BB address map is generated.
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# RUN: llc -mtriple x86_64-unknown-linux-gnu -start-after=bbsections-prepare %s -o - | FileCheck %s -check-prefix=CHECK
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# How to generate the input:
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# foo.cc
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# int foo(bool k) {
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# if (k) return 1;
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# return 0;
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# }
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#
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# clang -O0 -S -emit-llvm foo.cc
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# llc < foo.ll -stop-after=bbsections-prepare -basic-block-sections=labels
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# CHECK: .section .llvm_bb_addr_map,"o",@llvm_bb_addr_map,.text
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--- |
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; ModuleID = '<stdin>'
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source_filename = "/tmp/foo.cc"
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target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-pc-linux-gnu"
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; Function Attrs: mustprogress noinline nounwind optnone uwtable
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define dso_local noundef i32 @_Z3foob(i1 noundef zeroext %0) #0 {
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%2 = alloca i32, align 4
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%3 = alloca i8, align 1
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%4 = zext i1 %0 to i8
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store i8 %4, i8* %3, align 1
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%5 = load i8, i8* %3, align 1
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%6 = trunc i8 %5 to i1
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br i1 %6, label %7, label %8
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7: ; preds = %1
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store i32 1, i32* %2, align 4
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br label %9
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8: ; preds = %1
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store i32 0, i32* %2, align 4
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br label %9
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9: ; preds = %8, %7
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%10 = load i32, i32* %2, align 4
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ret i32 %10
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}
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attributes #0 = { mustprogress noinline nounwind optnone uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
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!llvm.module.flags = !{!0, !1, !2, !3, !4}
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!llvm.ident = !{!5}
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!0 = !{i32 1, !"wchar_size", i32 4}
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!1 = !{i32 7, !"PIC Level", i32 2}
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!2 = !{i32 7, !"PIE Level", i32 2}
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!3 = !{i32 7, !"uwtable", i32 1}
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!4 = !{i32 7, !"frame-pointer", i32 2}
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!5 = !{!"Debian clang version 14.0.6-2"}
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...
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---
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name: _Z3foob
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alignment: 16
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exposesReturnsTwice: false
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legalized: false
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regBankSelected: false
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selected: false
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failedISel: false
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tracksRegLiveness: true
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hasWinCFI: false
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callsEHReturn: false
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callsUnwindInit: false
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hasEHCatchret: false
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hasEHScopes: false
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hasEHFunclets: false
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failsVerification: false
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tracksDebugUserValues: true
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registers: []
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liveins:
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- { reg: '$edi', virtual-reg: '' }
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frameInfo:
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isFrameAddressTaken: false
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isReturnAddressTaken: false
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hasStackMap: false
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hasPatchPoint: false
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stackSize: 8
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offsetAdjustment: -8
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maxAlignment: 4
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adjustsStack: false
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hasCalls: false
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stackProtector: ''
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functionContext: ''
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maxCallFrameSize: 0
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cvBytesOfCalleeSavedRegisters: 0
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hasOpaqueSPAdjustment: false
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hasVAStart: false
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hasMustTailInVarArgFunc: false
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hasTailCall: false
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localFrameSize: 0
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savePoint: ''
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restorePoint: ''
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fixedStack:
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- { id: 0, type: spill-slot, offset: -16, size: 8, alignment: 16, stack-id: default,
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callee-saved-register: '', callee-saved-restored: true, debug-info-variable: '',
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debug-info-expression: '', debug-info-location: '' }
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stack:
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- { id: 0, name: '', type: default, offset: -24, size: 4, alignment: 4,
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stack-id: default, callee-saved-register: '', callee-saved-restored: true,
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debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
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- { id: 1, name: '', type: default, offset: -17, size: 1, alignment: 1,
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stack-id: default, callee-saved-register: '', callee-saved-restored: true,
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debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
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callSites: []
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debugValueSubstitutions: []
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constants: []
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machineFunctionInfo: {}
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body: |
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bb.0 (%ir-block.1, bb_id 0):
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successors: %bb.2(0x40000000), %bb.1(0x40000000)
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liveins: $edi
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frame-setup PUSH64r killed $rbp, implicit-def $rsp, implicit $rsp
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frame-setup CFI_INSTRUCTION def_cfa_offset 16
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frame-setup CFI_INSTRUCTION offset $rbp, -16
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$rbp = frame-setup MOV64rr $rsp
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frame-setup CFI_INSTRUCTION def_cfa_register $rbp
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renamable $dil = AND8ri renamable $dil, 1, implicit-def dead $eflags, implicit killed $edi, implicit-def $edi
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MOV8mr $rbp, 1, $noreg, -1, $noreg, renamable $dil, implicit killed $edi :: (store (s8) into %ir.3)
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TEST8mi $rbp, 1, $noreg, -1, $noreg, 1, implicit-def $eflags :: (load (s8) from %ir.3)
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JCC_1 %bb.2, 4, implicit killed $eflags
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bb.1 (%ir-block.7, bb_id 1):
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successors: %bb.3(0x80000000)
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MOV32mi $rbp, 1, $noreg, -8, $noreg, 1 :: (store (s32) into %ir.2)
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JMP_1 %bb.3
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bb.2 (%ir-block.8, bb_id 2):
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successors: %bb.3(0x80000000)
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MOV32mi $rbp, 1, $noreg, -8, $noreg, 0 :: (store (s32) into %ir.2)
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bb.3 (%ir-block.9, bb_id 3):
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renamable $eax = MOV32rm $rbp, 1, $noreg, -8, $noreg :: (load (s32) from %ir.2)
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$rbp = frame-destroy POP64r implicit-def $rsp, implicit $rsp
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frame-destroy CFI_INSTRUCTION def_cfa $rsp, 8
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RET64 implicit $eax
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...
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