Unverified Commit b6b49288 authored by Job Noorman's avatar Job Noorman Committed by GitHub
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[BOLT][RISCV] Set minimum function alignment to 2 for RVC (#69837) 

In #67707, the minimum function alignment on RISC-V was set to 4. When
RVC (compressed instructions) is enabled, the minimum alignment can be
reduced to 2.

This patch implements this by delegating the choice of minimum alignment
to a new `MCPlusBuilder::getMinFunctionAlignment` function. This way,
the target-dependent code in `BinaryFunction` is minimized.
parent 2ad9fde4

bolt/include/bolt/Core/BinaryFunction.h

+1 −8
Original line number Diff line number Diff line
@@ -1721,14 +1721,7 @@ public: 
    // Align data in code BFs minimum to CI alignment

    if (!size() && hasIslandsInfo())

      return getConstantIslandAlignment();



    // Minimal code alignment on AArch64 and RISCV is 4

    if (BC.isAArch64() || BC.isRISCV())

      return 4;



    // We have to use at least 2-byte alignment for functions because

    // of C++ ABI.

    return 2;

    return BC.MIB->getMinFunctionAlignment();

  }



  Align getMinAlign() const { return Align(getMinAlignment()); }

bolt/include/bolt/Core/MCPlusBuilder.h

+6 −0
Original line number Diff line number Diff line
@@ -2077,6 +2077,12 @@ public: 
    return BlocksVectorTy();

  }



  virtual uint16_t getMinFunctionAlignment() const {

    // We have to use at least 2-byte alignment for functions because of C++

    // ABI.

    return 2;

  }



  // AliasMap caches a mapping of registers to the set of registers that

  // alias (are sub or superregs of itself, including itself).

  std::vector<BitVector> AliasMap;

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

+2 −0
Original line number Diff line number Diff line
@@ -1643,6 +1643,8 @@ public: 


    return Relocation({RelOffset, RelSymbol, RelType, RelAddend, 0});

  }



  uint16_t getMinFunctionAlignment() const override { return 4; }

};



} // end anonymous namespace

bolt/lib/Target/RISCV/RISCVMCPlusBuilder.cpp

+8 −0
Original line number Diff line number Diff line
@@ -15,6 +15,7 @@ 
#include "bolt/Core/MCPlusBuilder.h"

#include "llvm/BinaryFormat/ELF.h"

#include "llvm/MC/MCInst.h"

#include "llvm/MC/MCSubtargetInfo.h"

#include "llvm/Support/Debug.h"

#include "llvm/Support/ErrorHandling.h"

#include "llvm/Support/Format.h"
@@ -490,6 +491,13 @@ public: 
    assert(Second.getOpcode() == RISCV::JALR);

    return true;

  }



  uint16_t getMinFunctionAlignment() const override {

    if (STI->hasFeature(RISCV::FeatureStdExtC) ||

        STI->hasFeature(RISCV::FeatureStdExtZca))

      return 2;

    return 4;

  }

};



} // end anonymous namespace

bolt/test/RISCV/function-alignment.s

0 → 100644
+38 −0
Original line number Diff line number Diff line 
## Test that BOLT uses a minimum function alignment of 4 (or 2 for RVC) bytes.



# RUN: llvm-mc -triple=riscv64 -filetype=obj -o %t.o %s

# RUN: ld.lld -q -o %t %t.o

# RUN: llvm-bolt --align-functions=1 --use-old-text=0 -o %t.bolt %t

# RUN: llvm-nm -n %t.bolt | FileCheck %s



# RUN: llvm-mc -triple=riscv64 -mattr=+c -filetype=obj -o %t-c.o %s

# RUN: ld.lld -q -o %t-c %t-c.o

# RUN: llvm-bolt --align-functions=1 --use-old-text=0 -o %t-c.bolt %t-c

# RUN: llvm-nm -n %t-c.bolt | FileCheck --check-prefix=CHECK-C %s



# CHECK:      {{[048c]}} T _start

# CHECK-NEXT: {{[048c]}} T dummy



# CHECK-C:      {{[02468ace]}} T _start

# CHECK-C-NEXT: {{[02468ace]}} T dummy



    .text



    # Make sure input binary is only 1 byte aligned. BOLT should increase the

    # alignment to 2 or 4 bytes.

    .byte 0

    .balign 1



    .globl _start

    .type _start, @function

_start:

    # Dummy reloc to force relocation mode.

    .reloc 0, R_RISCV_NONE

    ret

    .size _start, .-_start



    .globl dummy

    .type dummy, @function

dummy:

    ret

    .size dummy, .-dummy