From a5547e5b1d8cd0ab1ebc538a3dadfa140261e4cd Mon Sep 17 00:00:00 2001
From: Christian Prochaska <christian.prochaska@genode-labs.com>
Date: Mon, 10 Dec 2018 14:18:37 +0100
Subject: [PATCH] base: improve floating point output

Fixes #2876
---
 repos/base/recipes/pkg/test-log/runtime       |  1 +
 repos/base/run/log.run                        |  1 +
 repos/base/src/include/base/internal/output.h | 28 ++++++++++++++-----
 repos/base/src/test/log/main.cc               |  2 ++
 4 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/repos/base/recipes/pkg/test-log/runtime b/repos/base/recipes/pkg/test-log/runtime
index b1e7c4dc6..4f5469b5b 100644
--- a/repos/base/recipes/pkg/test-log/runtime
+++ b/repos/base/recipes/pkg/test-log/runtime
@@ -9,6 +9,7 @@
 			[init -> test-log] invalid hex range:  [f8,08) (overflow!)
 			[init -> test-log] negative hex char:  0xfe
 			[init -> test-log] positive hex char:  0x02
+			[init -> test-log] floating point:     1.70
 			[init -> test-log] multiarg string:    "parent -> child.7"
 			[init -> test-log] String(Hex(3)):     0x3
 			[init -> test-log] Very long messages:
diff --git a/repos/base/run/log.run b/repos/base/run/log.run
index 545809a61..5cabc9f12 100644
--- a/repos/base/run/log.run
+++ b/repos/base/run/log.run
@@ -36,6 +36,7 @@ compare_output_to {
 [init -> test-log] invalid hex range:  [f8,08) (overflow!)
 [init -> test-log] negative hex char:  0xfe
 [init -> test-log] positive hex char:  0x02
+[init -> test-log] floating point:     1.70
 [init -> test-log] multiarg string:    "parent -> child.7"
 [init -> test-log] String(Hex(3)):     0x3
 [init -> test-log] Very long messages:
diff --git a/repos/base/src/include/base/internal/output.h b/repos/base/src/include/base/internal/output.h
index 29a28b1a0..efbe551b8 100644
--- a/repos/base/src/include/base/internal/output.h
+++ b/repos/base/src/include/base/internal/output.h
@@ -114,13 +114,27 @@ static inline void out_float(T value, unsigned base, unsigned length, OUT_CHAR_F
 {
 	using namespace Genode;
 
+	/*
+	 * If the compiler decides to move a value from the FPU to the stack and
+	 * back, the value can slightly change because of different encodings. This
+	 * can cause problems if the value is assumed to stay the same in a chain
+	 * of calculations. For example, if casting 6.999 to int results in 6, this
+	 * number 6 needs to be subtracted from 6.999 in the next step and not from
+	 * 7 after an unexpected conversion, otherwise the next cast for a decimal
+	 * place would result in 10 instead of 9.
+	 * By storing the value in a volatile variable, the conversion step between
+	 * FPU and stack happens in a more deterministic way, which gives more
+	 * consistent results with this function.
+	 */
+	T volatile volatile_value = value;
+
 	/* set flag if value is negative */
-	int neg = value < 0 ? 1 : 0;
+	int neg = volatile_value < 0 ? 1 : 0;
 
 	/* get absolute value */
-	value = value < 0 ? -value : value;
+	volatile_value = volatile_value < 0 ? -volatile_value : volatile_value;
 
-	uint64_t integer = (uint64_t)value;
+	uint64_t integer = (uint64_t)volatile_value;
 
 	if (neg)
 		out_char('-');
@@ -130,14 +144,14 @@ static inline void out_float(T value, unsigned base, unsigned length, OUT_CHAR_F
 
 	if (length) {
 		do {
-			value -= integer;
-			value = value*base;
+			volatile_value -= integer;
+			volatile_value = volatile_value*base;
 
-			integer = (int64_t)value;
+			integer = (int64_t)volatile_value;
 			out_char(ascii(integer));
 
 			length--;
-		} while (length && (value > 0.0));
+		} while (length && (volatile_value > 0.0));
 	}
 }
 
diff --git a/repos/base/src/test/log/main.cc b/repos/base/src/test/log/main.cc
index 034f1d09a..a07a36a78 100644
--- a/repos/base/src/test/log/main.cc
+++ b/repos/base/src/test/log/main.cc
@@ -28,6 +28,8 @@ void Component::construct(Genode::Env &env)
 	log("negative hex char:  ", Hex((char)-2LL, Hex::PREFIX, Hex::PAD));
 	log("positive hex char:  ", Hex((char) 2LL, Hex::PREFIX, Hex::PAD));
 
+	log("floating point:     ", 1700.0 / 1000);
+
 	typedef String<128> Label;
 	log("multiarg string:    ", Label(Char('"'), "parent -> child.", 7, Char('"')));