[PATCH] D32907: [ELF] - Fix warnings when LLD compiled using gcc 7.1.0

[Rafael Avila de Espindola via llvm-commits]

Can you use LLVM_FALLTHROUGH?


George Rimar via Phabricator <reviews at reviews.llvm.org> writes:

> grimar created this revision.
> Herald added subscribers: rengolin, aemerson.
>
> I tried to compile LLD using gcc 7.1.0 and got 2 warnings:
>
>   [1/3] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/Target.cpp.o
>   /home/umb/LLVM/llvm/tools/lld/ELF/Target.cpp: In member function ‘virtual void lld::elf::{anonymous}::ARMTargetInfo::relocateOne(uint8_t*, uint32_t, uint64_t) const’:
>   /home/umb/LLVM/llvm/tools/lld/ELF/Target.cpp:1897:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
>        if ((read32le(Loc) & 0xfe000000) == 0xfa000000)
>        ^~
>   /home/umb/LLVM/llvm/tools/lld/ELF/Target.cpp:1903:3: note: here
>      case R_ARM_JUMP24:
>      ^~~~
>   /home/umb/LLVM/llvm/tools/lld/ELF/Target.cpp:1935:14: warning: this statement may fall through [-Wimplicit-fallthrough=]
>        write16le(Loc + 2, (read16le(Loc + 2) & ~0x1000) | (Val & 1) << 12);
>        ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   /home/umb/LLVM/llvm/tools/lld/ELF/Target.cpp:1938:3: note: here
>      case R_ARM_THM_JUMP24:
>      ^~~~
>
> Them can be fixed easily. GCC wants to see "fallthrough" word in a comment that has nothing else. People in the web also uses "/* FALLTHRU */" comment for that. Way used in this patch looks to be consistent with rest LLVM code. Patch fixes the warnings for me.
>
>
> https://reviews.llvm.org/D32907
>
> Files:
>   ELF/Target.cpp
>
>
> Index: ELF/Target.cpp
> ===================================================================
> --- ELF/Target.cpp
> +++ ELF/Target.cpp
> @@ -1898,7 +1898,8 @@
>        // BLX (always unconditional) instruction to an ARM Target, select an
>        // unconditional BL.
>        write32le(Loc, 0xeb000000 | (read32le(Loc) & 0x00ffffff));
> -  // fall through as BL encoding is shared with B
> +  // fallthrough
> +  // as BL encoding is shared with B
>    case R_ARM_JUMP24:
>    case R_ARM_PC24:
>    case R_ARM_PLT32:
> @@ -1932,7 +1933,8 @@
>      }
>      // Bit 12 is 0 for BLX, 1 for BL
>      write16le(Loc + 2, (read16le(Loc + 2) & ~0x1000) | (Val & 1) << 12);
> -  // Fall through as rest of encoding is the same as B.W
> +  // fallthrough
> +  // as rest of encoding is the same as B.W
>    case R_ARM_THM_JUMP24:
>      // Encoding B  T4, BL T1, BLX T2: Val = S:I1:I2:imm10:imm11:0
>      // FIXME: Use of I1 and I2 require v6T2ops
>
>
> Index: ELF/Target.cpp
> ===================================================================
> --- ELF/Target.cpp
> +++ ELF/Target.cpp
> @@ -1898,7 +1898,8 @@
>        // BLX (always unconditional) instruction to an ARM Target, select an
>        // unconditional BL.
>        write32le(Loc, 0xeb000000 | (read32le(Loc) & 0x00ffffff));
> -  // fall through as BL encoding is shared with B
> +  // fallthrough
> +  // as BL encoding is shared with B
>    case R_ARM_JUMP24:
>    case R_ARM_PC24:
>    case R_ARM_PLT32:
> @@ -1932,7 +1933,8 @@
>      }
>      // Bit 12 is 0 for BLX, 1 for BL
>      write16le(Loc + 2, (read16le(Loc + 2) & ~0x1000) | (Val & 1) << 12);
> -  // Fall through as rest of encoding is the same as B.W
> +  // fallthrough
> +  // as rest of encoding is the same as B.W
>    case R_ARM_THM_JUMP24:
>      // Encoding B  T4, BL T1, BLX T2: Val = S:I1:I2:imm10:imm11:0
>      // FIXME: Use of I1 and I2 require v6T2ops