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gallivm: Use a faster (and less accurate) log2 in lod computation.

This commit is contained in:
José Fonseca 2010-10-06 14:06:14 +01:00
parent df3505b193
commit 4648846bd6
3 changed files with 53 additions and 0 deletions
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44
src/gallium/auxiliary/gallivm/lp_bld_arit.c
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@ -2286,3 +2286,47 @@ lp_build_log2(struct lp_build_context *bld,
lp_build_log2_approx(bld, x, NULL, NULL, &res);
return res;
}
/**
* Faster (and less accurate) log2.
*
* log2(x) = floor(log2(x)) + frac(x)
*
* See http://www.flipcode.com/archives/Fast_log_Function.shtml
*/
LLVMValueRef
lp_build_fast_log2(struct lp_build_context *bld,
LLVMValueRef x)
{
const struct lp_type type = bld->type;
LLVMTypeRef vec_type = bld->vec_type;
LLVMTypeRef int_vec_type = bld->int_vec_type;
unsigned mantissa = lp_mantissa(type);
LLVMValueRef mantmask = lp_build_const_int_vec(type, (1ULL << mantissa) - 1);
LLVMValueRef one = LLVMConstBitCast(bld->one, int_vec_type);
LLVMValueRef ipart;
LLVMValueRef fpart;
assert(lp_check_value(bld->type, x));
assert(type.floating);
x = LLVMBuildBitCast(bld->builder, x, int_vec_type, "");
/* ipart = floor(log2(x)) - 1 */
ipart = LLVMBuildLShr(bld->builder, x, lp_build_const_int_vec(type, mantissa), "");
ipart = LLVMBuildAnd(bld->builder, ipart, lp_build_const_int_vec(type, 255), "");
ipart = LLVMBuildSub(bld->builder, ipart, lp_build_const_int_vec(type, 128), "");
ipart = LLVMBuildSIToFP(bld->builder, ipart, vec_type, "");
/* fpart = 1.0 + frac(x) */
fpart = LLVMBuildAnd(bld->builder, x, mantmask, "");
fpart = LLVMBuildOr(bld->builder, fpart, one, "");
fpart = LLVMBuildBitCast(bld->builder, fpart, vec_type, "");
/* floor(log2(x)) + frac(x) */
return LLVMBuildFAdd(bld->builder, ipart, fpart, "");
}

5
src/gallium/auxiliary/gallivm/lp_bld_arit.h
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@ -212,6 +212,11 @@ LLVMValueRef
lp_build_log2(struct lp_build_context *bld,
LLVMValueRef a);
LLVMValueRef
lp_build_fast_log2(struct lp_build_context *bld,
LLVMValueRef a);
void
lp_build_exp2_approx(struct lp_build_context *bld,
LLVMValueRef x,

4
src/gallium/auxiliary/gallivm/lp_bld_sample.c
View File

@ -243,7 +243,11 @@ lp_build_lod_selector(struct lp_build_sample_context *bld,
}
/* compute lod = log2(rho) */
#if 0
lod = lp_build_log2(float_bld, rho);
#else
lod = lp_build_fast_log2(float_bld, rho);
#endif
/* add shader lod bias */
if (lod_bias) {