Refactor extendsfdf2.

Patch by: GuanHong Liu
Differential Revision: http://reviews.llvm.org/D3887


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209781 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 106 insertions, 0 deletions

diff --git a/lib/builtins/fp_extend_impl.inc b/lib/builtins/fp_extend_impl.inc
new file mode 100644
index 000000000..f6953ff4b
--- /dev/null
+++ b/lib/builtins/fp_extend_impl.inc
@@ -0,0 +1,106 @@
+//=-lib/fp_extend_impl.inc - low precision -> high precision conversion -*-- -//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a fairly generic conversion from a narrower to a wider
+// IEEE-754 floating-point type.  The constants and types defined following the
+// includes below parameterize the conversion.
+//
+// It does not support types that don't use the usual IEEE-754 interchange
+// formats; specifically, some work would be needed to adapt it to
+// (for example) the Intel 80-bit format or PowerPC double-double format.
+//
+// Note please, however, that this implementation is only intended to support
+// *widening* operations; if you need to convert to a *narrower* floating-point
+// type (e.g. double -> float), then this routine will not do what you want it
+// to.
+//
+// It also requires that integer types at least as large as both formats
+// are available on the target platform; this may pose a problem when trying
+// to add support for quad on some 32-bit systems, for example.  You also may
+// run into trouble finding an appropriate CLZ function for wide source types;
+// you will likely need to roll your own on some platforms.
+//
+// Finally, the following assumptions are made:
+//
+// 1. floating-point types and integer types have the same endianness on the
+//    target platform
+//
+// 2. quiet NaNs, if supported, are indicated by the leading bit of the
+//    significand field being set
+//
+//===----------------------------------------------------------------------===//
+
+#include "fp_extend.h"
+
+static inline dst_t __extendXfYf2__(src_t a) {
+    // Various constants whose values follow from the type parameters.
+    // Any reasonable optimizer will fold and propagate all of these.
+    const int srcBits = sizeof(src_t)*CHAR_BIT;
+    const int srcExpBits = srcBits - srcSigBits - 1;
+    const int srcInfExp = (1 << srcExpBits) - 1;
+    const int srcExpBias = srcInfExp >> 1;
+
+    const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
+    const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
+    const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
+    const src_rep_t srcAbsMask = srcSignMask - 1;
+    const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
+    const src_rep_t srcNaNCode = srcQNaN - 1;
+
+    const int dstBits = sizeof(dst_t)*CHAR_BIT;
+    const int dstExpBits = dstBits - dstSigBits - 1;
+    const int dstInfExp = (1 << dstExpBits) - 1;
+    const int dstExpBias = dstInfExp >> 1;
+
+    const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits;
+
+    // Break a into a sign and representation of the absolute value
+    const src_rep_t aRep = srcToRep(a);
+    const src_rep_t aAbs = aRep & srcAbsMask;
+    const src_rep_t sign = aRep & srcSignMask;
+    dst_rep_t absResult;
+
+    if (aAbs - srcMinNormal < srcInfinity - srcMinNormal) {
+        // a is a normal number.
+        // Extend to the destination type by shifting the significand and
+        // exponent into the proper position and rebiasing the exponent.
+        absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits);
+        absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits;
+    }
+
+    else if (aAbs >= srcInfinity) {
+        // a is NaN or infinity.
+        // Conjure the result by beginning with infinity, then setting the qNaN
+        // bit (if needed) and right-aligning the rest of the trailing NaN
+        // payload field.
+        absResult = (dst_rep_t)dstInfExp << dstSigBits;
+        absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits);
+        absResult |= (dst_rep_t)(aAbs & srcNaNCode) << (dstSigBits - srcSigBits);
+    }
+
+    else if (aAbs) {
+        // a is denormal.
+        // renormalize the significand and clear the leading bit, then insert
+        // the correct adjusted exponent in the destination type.
+        const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal);
+        absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale);
+        absResult ^= dstMinNormal;
+        const int resultExponent = dstExpBias - srcExpBias - scale + 1;
+        absResult |= (dst_rep_t)resultExponent << dstSigBits;
+    }
+
+    else {
+        // a is zero.
+        absResult = 0;
+    }
+
+    // Apply the signbit to (dst_t)abs(a).
+    const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits);
+    return dstFromRep(result);
+}