From: José Fonseca Date: Wed, 13 Oct 2010 14:45:24 +0000 (+0100) Subject: gallivm: More accurate float -> 24bit & 32bit unorm conversion. X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=60c5d4735d5fa5642c84f6d7c3847ac213efcb53;p=mesa.git gallivm: More accurate float -> 24bit & 32bit unorm conversion. --- diff --git a/src/gallium/auxiliary/gallivm/lp_bld_conv.c b/src/gallium/auxiliary/gallivm/lp_bld_conv.c index 20aa93e7781..6967dd26225 100644 --- a/src/gallium/auxiliary/gallivm/lp_bld_conv.c +++ b/src/gallium/auxiliary/gallivm/lp_bld_conv.c @@ -97,58 +97,104 @@ lp_build_clamped_float_to_unsigned_norm(LLVMBuilderRef builder, LLVMTypeRef int_vec_type = lp_build_int_vec_type(src_type); LLVMValueRef res; unsigned mantissa; - unsigned n; - unsigned long long ubound; - unsigned long long mask; - double scale; - double bias; assert(src_type.floating); + assert(dst_width <= src_type.width); + src_type.sign = FALSE; mantissa = lp_mantissa(src_type); - /* We cannot carry more bits than the mantissa */ - n = MIN2(mantissa, dst_width); + if (dst_width <= mantissa) { + /* + * Apply magic coefficients that will make the desired result to appear + * in the lowest significant bits of the mantissa, with correct rounding. + * + * This only works if the destination width fits in the mantissa. + */ - /* This magic coefficients will make the desired result to appear in the - * lowest significant bits of the mantissa. - */ - ubound = ((unsigned long long)1 << n); - mask = ubound - 1; - scale = (double)mask/ubound; - bias = (double)((unsigned long long)1 << (mantissa - n)); + unsigned long long ubound; + unsigned long long mask; + double scale; + double bias; + + ubound = (1ULL << dst_width); + mask = ubound - 1; + scale = (double)mask/ubound; + bias = (double)(1ULL << (mantissa - dst_width)); + + res = LLVMBuildFMul(builder, src, lp_build_const_vec(src_type, scale), ""); + res = LLVMBuildFAdd(builder, res, lp_build_const_vec(src_type, bias), ""); + res = LLVMBuildBitCast(builder, res, int_vec_type, ""); + res = LLVMBuildAnd(builder, res, lp_build_const_int_vec(src_type, mask), ""); + } + else if (dst_width == (mantissa + 1)) { + /* + * The destination width matches exactly what can be represented in + * floating point (i.e., mantissa + 1 bits). So do a straight + * multiplication followed by casting. No further rounding is necessary. + */ - res = LLVMBuildFMul(builder, src, lp_build_const_vec(src_type, scale), ""); - res = LLVMBuildFAdd(builder, res, lp_build_const_vec(src_type, bias), ""); - res = LLVMBuildBitCast(builder, res, int_vec_type, ""); + double scale; - if(dst_width > n) { - int shift = dst_width - n; - res = LLVMBuildShl(builder, res, lp_build_const_int_vec(src_type, shift), ""); + scale = (double)((1ULL << dst_width) - 1); - /* TODO: Fill in the empty lower bits for additional precision? */ - /* YES: this fixes progs/trivial/tri-z-eq.c. - * Otherwise vertex Z=1.0 values get converted to something like - * 0xfffffb00 and the test for equality with 0xffffffff fails. + res = LLVMBuildFMul(builder, src, lp_build_const_vec(src_type, scale), ""); + res = LLVMBuildFPToSI(builder, res, int_vec_type, ""); + } + else { + /* + * The destination exceeds what can be represented in the floating point. + * So multiply by the largest power two we get away with, and when + * subtract the most significant bit to rescale to normalized values. + * + * The largest power of two factor we can get away is + * (1 << (src_type.width - 1)), because we need to use signed . In theory it + * should be (1 << (src_type.width - 2)), but IEEE 754 rules states + * INT_MIN should be returned in FPToSI, which is the correct result for + * values near 1.0! + * + * This means we get (src_type.width - 1) correct bits for values near 0.0, + * and (mantissa + 1) correct bits for values near 1.0. Equally or more + * important, we also get exact results for 0.0 and 1.0. */ -#if 0 - { - LLVMValueRef msb; - msb = LLVMBuildLShr(builder, res, lp_build_const_int_vec(src_type, dst_width - 1), ""); - msb = LLVMBuildShl(builder, msb, lp_build_const_int_vec(src_type, shift), ""); - msb = LLVMBuildSub(builder, msb, lp_build_const_int_vec(src_type, 1), ""); - res = LLVMBuildOr(builder, res, msb, ""); - } -#elif 0 - while(shift > 0) { - res = LLVMBuildOr(builder, res, LLVMBuildLShr(builder, res, lp_build_const_int_vec(src_type, n), ""), ""); - shift -= n; - n *= 2; + + unsigned n = MIN2(src_type.width - 1, dst_width); + + double scale = (double)(1ULL << n); + unsigned lshift = dst_width - n; + unsigned rshift = n; + LLVMValueRef lshifted; + LLVMValueRef rshifted; + + res = LLVMBuildFMul(builder, src, lp_build_const_vec(src_type, scale), ""); + res = LLVMBuildFPToSI(builder, res, int_vec_type, ""); + + /* + * Align the most significant bit to its final place. + * + * This will cause 1.0 to overflow to 0, but the later adjustment will + * get it right. + */ + if (lshift) { + lshifted = LLVMBuildShl(builder, res, + lp_build_const_int_vec(src_type, lshift), ""); + } else { + lshifted = res; } -#endif + + /* + * Align the most significant bit to the right. + */ + rshifted = LLVMBuildAShr(builder, res, + lp_build_const_int_vec(src_type, rshift), ""); + + /* + * Subtract the MSB to the LSB, therefore re-scaling from + * (1 << dst_width) to ((1 << dst_width) - 1). + */ + + res = LLVMBuildSub(builder, lshifted, rshifted, ""); } - else - res = LLVMBuildAnd(builder, res, lp_build_const_int_vec(src_type, mask), ""); return res; }