llvm dev - Sep 2011 - [LLVMdev] Disabling certain optimizations in llvm-gcc and llc

Hi all,

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?

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?



Thanks a lot in advance!

Harel Cain
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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.

Hello
> 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. It's performed even before IR emission.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University