[llvm-commits] [PATCH] optimise constant arguments in LowerIntrinsicCall()

[Chris Lattner]

On May 7, 2009, at 6:28 AM, Jay Foad wrote:

> Given this source:
>
> void f(int *p) {
>    p[0] = p[1] = p[2] = p[3] = p[4] = p[5] = p[6] = p[7] = p[8] =  
> p[9] = 0;
> }
>
> if I compile with optimisation and then run it through the C backend:

Looks good to me, please apply.  Thanks Jay,

-Chris

>
>
> $ .../llvm-gcc4.2-2.5-x86-linux-RHEL4/bin/llvm-gcc -emit-llvm -O2 -c  
> m.c
> $ llc -march=c -o - m.o
>
> I get:
>
> void f(unsigned int *llvm_cbe_p) {
>  unsigned char *ltmp_0_1;
>
>  *llvm_cbe_p = 0u;
>  ltmp_0_1 = memset((((unsigned char *)((&llvm_cbe_p[((signed int
> )1u)])))), (((unsigned int )(unsigned char )((unsigned char )0))),
> (((unsigned int )36ull)));
>  return;
> }
>
> The second and third arguments in the call to memset() involve
> unnecessary casts of a constant value. This happens because
> LowerIntrinsicCall() turns:
>
> 	call void @llvm.memset.i64(i8* %1, i8 0, i64 36, i32 4)
>
> into:
>
>        %2 = trunc i64 36 to i32                ; <i32> [#uses=1]
>        %3 = zext i8 0 to i32           ; <i32> [#uses=1]
>        %4 = call i8* @memset(i8* %1, i32 %3, i32 %2)           ; <i8*>
>
> and nothing later constant-folds the trunc and zext instructions.
>
> The attached patch addresses this, by optimising LowerIntrinsicCall()
> for the case of constant arguments. With this patch applied, the C
> backend produces:
>
>  ltmp_0_1 = memset((((unsigned char *)((&llvm_cbe_p[((signed int
> )1u)])))), 0u, 36u);
>
> The patch passes "make check".
>
> Alternatively, maybe some other optimisation pass could be scheduled
> that constant-folds the trunc and zext instructions, but this seems
> like overkill to me.
>
> Incidentally, it seems a bit odd that llvm-gcc generates
> llvm.memset.i64 instead of llvm.memset.i32 in the first place, but I
> haven't investigated why this happens.
>
> Thanks,
> Jay.
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