From dc48c4fff4fdb61ebad48b62ae1f21a32af1e50c Mon Sep 17 00:00:00 2001
From: Jordan Rupprecht <rupprecht@google.com>
Date: Mon, 24 Sep 2018 20:39:19 +0000
Subject: [compiler-rt] [builtins] Add logb/logbf/logbl methods to compiler-rt
 to avoid libm dependencies when possible.

Summary:
The complex division builtins (div?c3) use logb methods from libm to scale numbers during division and avoid rounding issues. However, these come from libm, meaning anyone that uses --rtlib=compiler-rt also has to include -lm. Implement logb* methods for standard ieee 754 floats so we can avoid -lm on those platforms, falling back to the old behavior (using either logb() or `__builtin_logb()`) when not supported.

These new methods are defined internally as `__compiler_rt_logb` so as not to conflict with the libm definitions in any way.

This fixes just the libm methods mentioned in PR32279 and PR28652. libc is still required, although that seems to not be an issue.

Note: this is proposed as an alternative to just adding -lm: D49330.

Reviewers: efriedma, compnerd, scanon, echristo

Reviewed By: echristo

Subscribers: jsji, echristo, nemanjai, dberris, mgorny, kbarton, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49514

llvm-svn: 342917
---
 .../test/builtins/Unit/compiler_rt_logb_test.c     | 63 ++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 compiler-rt/test/builtins/Unit/compiler_rt_logb_test.c

(limited to 'compiler-rt/test/builtins/Unit/compiler_rt_logb_test.c')

diff --git a/compiler-rt/test/builtins/Unit/compiler_rt_logb_test.c b/compiler-rt/test/builtins/Unit/compiler_rt_logb_test.c
new file mode 100644
index 00000000000..8bab4ab083a
--- /dev/null
+++ b/compiler-rt/test/builtins/Unit/compiler_rt_logb_test.c
@@ -0,0 +1,63 @@
+// RUN: %clang_builtins %s %librt -lm -o %t && %run %t
+//===-- compiler_rt_logb_test.c - Test __compiler_rt_logb -----------------===//
+//
+//                     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 checks __compiler_rt_logb from the compiler_rt library for
+// conformance against libm.
+//
+//===----------------------------------------------------------------------===//
+
+#define DOUBLE_PRECISION
+#include <math.h>
+#include <stdio.h>
+#include "fp_lib.h"
+
+int test__compiler_rt_logb(fp_t x) {
+  fp_t crt_value = __compiler_rt_logb(x);
+  fp_t libm_value = logb(x);
+  // Compare actual rep, e.g. to avoid NaN != the same NaN
+  if (toRep(crt_value) != toRep(libm_value)) {
+    printf("error: in __compiler_rt_logb(%a [%lX]) = %a [%lX] !=  %a [%lX]\n",
+           x, toRep(x), crt_value, toRep(crt_value), libm_value,
+           toRep(libm_value));
+    return 1;
+  }
+  return 0;
+}
+
+double cases[] = {
+    1.e-6, -1.e-6, NAN, -NAN, INFINITY, -INFINITY, -1,
+    -0.0,  0.0,    1,   -2,   2,        -0.5,      0.5,
+};
+
+int main() {
+  const unsigned N = sizeof(cases) / sizeof(cases[0]);
+  unsigned i;
+  for (i = 0; i < N; ++i) {
+    if (test__compiler_rt_logb(cases[i])) return 1;
+  }
+
+  // Test a moving 1 bit, especially to handle denormal values.
+  // Test the negation as well.
+  rep_t x = signBit;
+  while (x) {
+    if (test__compiler_rt_logb(fromRep(x))) return 1;
+    if (test__compiler_rt_logb(fromRep(signBit ^ x))) return 1;
+    x >>= 1;
+  }
+  // Also try a couple moving ones
+  x = signBit | (signBit >> 1) | (signBit >> 2);
+  while (x) {
+    if (test__compiler_rt_logb(fromRep(x))) return 1;
+    if (test__compiler_rt_logb(fromRep(signBit ^ x))) return 1;
+    x >>= 1;
+  }
+
+  return 0;
+}
-- 
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