test: accel_cal: fix for gitlab

Fix a rounding error. See documentation for details. It is important
to include the detailed documentation as it provides strong details
for why these flags are necessary. It will avoid confusion in the
future.

BRANCH=none
BUG=b:178731498
TEST=zmake configure --test zephyr/test/accel_cal (both in chroot and
     in gitlab docker).

Change-Id: Ie040f49dfaacb84dc10b76481a16213be6a2f09e
Signed-off-by: Yuval Peress <peress@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3139254
Reviewed-by: Jeremy Bettis <jbettis@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

1 files changed, 42 insertions, 0 deletions

diff --git a/zephyr/fpu.cmake b/zephyr/fpu.cmake
new file mode 100644
index 0000000000..ddf782e5c8
--- /dev/null
+++ b/zephyr/fpu.cmake
@@ -0,0 +1,42 @@
+# Copyright 2021 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+#[[
+Disclaimer: the following example is pieced together from
+https://lemire.me/blog/2020/06/26/gcc-not-nearest/ along with other information
+found in GCC documentation.
+
+The following flags are needed to to ensure consistent FPU rounding in unit
+tests. For example using GNU GCC 7.5 rounds down. Note that at the time of
+writing, Clang 13.0.0 passes all the FPU unit tests without these flags.
+
+Some of the sensor logic which requires FPU support is susceptible to rounding
+errors. In GCC 7.5, as an example:
+
+  double x = 50178230318.0;
+  double y = 100000000000.0;
+  double ratio = x/y;
+
+In this example, we would expect ratio to be 0.50178230318. Instead, using a
+64-bit float, it falls between:
+* The floating-point number 0.501782303179999944 == 4519653187245114  * 2 ** -53
+* The floating-point number 0.501782303180000055 == 4519653187245115  * 2 ** -53
+
+The real mantissa using the same logic should be:
+0.50178230318 = 4519653187245114.50011795456 * 2 ** -53
+
+Since the mantissa's decimal is just over 0.5, it should stand to reason that
+the correct solution is 0.501782303180000055. To force GCC to round correctly
+we must set the following modes:
+1. 'sse' - Generate SSE instructions.
+2. 'fpmath=sse' - Use scalar floating-point instructions present in the SSE
+   instruction set.
+3. 'arch=pentium4' - Choose the Pentium4 because it's a stable choice that
+   supports SSE and is known to work (there may be other good choices).
+]]
+if(DEFINED CONFIG_SOC_POSIX)
+  set(FLOAT_FLAGS "-msse -mfpmath=sse -march=pentium4")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${FLOAT_FLAGS}")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${FLOAT_FLAGS}")
+endif()