possible floating point regression on ppc

[NattyNarwhal]

Introduced in #15930.

Steps to Reproduce

do something with floats, apparently

Current Behavior

looks like the high bit isn't doing what you expect

Expected Behavior

normal FP operations

On which platforms did you notice this

[ ] macOS
[ ] Linux
[ ] Windows
[X] AIX, i

(but it's probably a mini-ppc thing, don't have ppc64 linux to test with)

Version Used:

master

Stacktrace

exceptions test with more data (uint conversion left, double right)

Test run: image=/home/Calvin/src/mono/mono/mini/exceptions.exe, opts=
 ** test_0_simple_double_casts t1: 2147483647 (7FFFFFFF) 4294967295
Test 'test_0_simple_double_casts' failed result (got 1, expected 0).

basic-float (same)

Test run: image=/home/Calvin/src/mono/mono/mini/basic-float.exe, opts=
 ** test_0_fconv_u4 t1: 2147483648 (80000000) NaN
Test 'test_0_fconv_u4' failed result (got 1, expected 0).
 ** test_0_rconv_u4 t1: 2147483648 (80000000) NaN
Test 'test_0_rconv_u4' failed result (got 1, expected 0).
Results: total tests: 58, failed: 2, cfailed: 0 (pass: 96.55%)

then this from CI (likely more, but this jumped out)

Tests run: 2405, Passed: 2053, Errors: 0, Failures: 1, Inconclusive: 0
  Not run: 351, Invalid: 0, Ignored: 351, Skipped: 0
Elapsed time: 00:00:13.1450000

Errors and Failures:

1) SqlDoubleToSqlDecimal (MonoTests.System.Data.SqlTypes.SqlDecimalTest.SqlDoubleToSqlDecimal)
     #U01
  Expected: 120000000000m
  But was:  118111600639m

  at MonoTests.System.Data.SqlTypes.SqlDecimalTest.SqlDoubleToSqlDecimal () [0x00010] in /home/jenkins/workspace/test-mono-mainline-community/label/aix-ppc64/mcs/class/System.Data/Test/System.Data.SqlTypes/SqlDecimalTest.cs:634 
  at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
  at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0007c] in /home/jenkins/workspace/test-mono-mainline-community/label/aix-ppc64/mcs/class/corlib/System.Reflection/RuntimeMethodInfo.cs:395 

[filipnavara]

Please try adding #define MONO_ARCH_EMULATE_FCONV_TO_U4 1 in mono/mini/mini-ppc.h after the line with MONO_ARCH_EMULATE_FCONV_TO_U8. That should hopefully fix the basic-float tests.

[NattyNarwhal]

Yes, that fixes them.