[LLVMdev] Disabling certain optimizations in llvm-gcc and llc

[Duncan Sands]

Hi Harel,

> I'm trying to prevent two things from happening:
>
> 1. Intrinsic memcpy calls of for example 16 bytes in the IR code being
> translated into movl commands in x86 assembly code, for example, this code:
>          call void @llvm.memcpy.p0i8.p0i8.i32(i8* %3, i8* getelementptr inbounds
> ([16 x i8]* @.str1, i32 0, i32 0), i32 16, i32 1, i1 false)
> translated into this x86 assembly
>          movl    $6384738, 12(%ecx)      # imm = 0x616C62
>          movl    $543452274, 8(%ecx)     # imm = 0x20646C72
>          movl    $1870078063, 4(%ecx)    # imm = 0x6F77206F
>          movl    $1819043176, (%ecx)     # imm = 0x6C6C6568
> this happens even if I run llc -O0. Any way to prevent it, instead calling
> memcpy in the assembly listing?

-fno-builtin

> 2. sizeof's being resolved to literals already at the llvm-gcc front end stage
> when emitting IR, no matter if I use -O0 flag Is there any way to prevent it?

No.  The GCC parts of llvm-gcc fold sizeof to a constant long before it hits
the LLVM parts.

Best wishes, Duncan.