[builtins] Omit 80-bit builtins on Android and MSVC

[rprichard]

Summary

long double is a 64-bit double-precision type on:

• MSVC (32- and 64-bit x86)
• Android (32-bit x86)

long double is a 128-bit quad-precision type on x86_64 Android.

The assembly variants of the 80-bit builtins are correct, but some of
the builtins are implemented in C and require that long double be the
80-bit type passed via an x87 register.

[rprichard]

I'm suspicious of divtc3.c (in GENERIC_SOURCES) and multc3.c (in GENERIC_TF_SOURCES). They use the long double type without checking a preprocessor macro to ensure that long double is a 128-bit type. On 32-bit Android and MSVC, the type will be 64 bits, and on host GNU/Linux x86-{32,64}, the type will be 80 bits.

I'm not sure if tf/tc are always supposed to indicate the 128-bit IEEE 754 quad-precision binary128 type. e.g. On PowerPC64, it looks like tf/tc are the 128-bit double-double type instead.

Maybe on x86, though, the convention is that tf/tc is binary128 and xf/xc is 80-bit.

[compnerd]

This is definitely the right thing to do. I think that there is a more elegant way to write this, but that feels outside the scope of this change.

[rprichard]

Rebase this revision on top of the cleanup revision. (git rebase was
able to do the rebase automatically.)

[rprichard]

Is there anyone I should add as reviewer for the MSVC configuration?

compiler-rt/lib/builtins/CMakeLists.txt

set(GENERIC_SOURCES
   udivmodti4.c
   udivsi3.c
   udivti3.c
   umoddi3.c
   umodsi3.c
   umodti3.c
 )
 
+# TODO: Several "tf" files (and divtc3.c, but not multc3.c) are in
+# GENERIC_SOURCES instead of here.
 set(GENERIC_TF_SOURCES
   addtf3.c
   comparetf2.c
   divtc3.c
   divtf3.c
   extenddftf2.c
   extendsftf2.c
   fixtfdi.c

 
 if (NOT FUCHSIA)
   set(GENERIC_SOURCES
     ${GENERIC_SOURCES}
     clear_cache.c
   )
 endif()
 
-# These sources work on all x86 variants, but only x86 variants.
+# These files are used on 32-bit and 64-bit x86.
+set(x86_ARCH_SOURCES cpu_model.c)
+
+if (NOT MSVC)
   set(x86_ARCH_SOURCES
-  cpu_model.c
+    ${x86_ARCH_SOURCES}
+    i386/fp_mode.c
+  )
+endif ()
+
+# Implement extended-precision builtins, assuming long double is 80 bits.
+# long double is not 80 bits on Android or MSVC.
+set(x86_80_BIT_SOURCES
   divxc3.c
   fixxfdi.c
   fixxfti.c
   fixunsxfdi.c
   fixunsxfsi.c
   fixunsxfti.c
   floatdixf.c
   floattixf.c
   floatundixf.c
   floatuntixf.c
   mulxc3.c
   powixf2.c
 )
 
 if (NOT MSVC)
-  set(x86_ARCH_SOURCES
-    ${x86_ARCH_SOURCES}
-    i386/fp_mode.c
-  )
-endif ()
-
-if (NOT MSVC)
   set(x86_64_SOURCES
     ${GENERIC_SOURCES}
     ${GENERIC_TF_SOURCES}
     ${x86_ARCH_SOURCES}
     x86_64/floatdidf.c
     x86_64/floatdisf.c
-    x86_64/floatdixf.c
     x86_64/floatundidf.S
     x86_64/floatundisf.S
+  )
+
+  if (NOT ANDROID)
+    set(x86_64_SOURCES
+      ${x86_64_SOURCES}
+      ${x86_80_BIT_SOURCES}
+      x86_64/floatdixf.c
       x86_64/floatundixf.S
     )
+  endif()
 
   # Darwin x86_64 Haswell
   set(x86_64h_SOURCES ${x86_64_SOURCES})
 
   if (WIN32)
     set(x86_64_SOURCES
       ${x86_64_SOURCES}
       x86_64/chkstk.S
       x86_64/chkstk2.S
     )
   endif()
 
   set(i386_SOURCES
     ${GENERIC_SOURCES}
     ${x86_ARCH_SOURCES}
     i386/ashldi3.S
     i386/ashrdi3.S
     i386/divdi3.S
     i386/floatdidf.S
     i386/floatdisf.S
-    i386/floatdixf.S
     i386/floatundidf.S
     i386/floatundisf.S
-    i386/floatundixf.S
     i386/lshrdi3.S
     i386/moddi3.S
     i386/muldi3.S
     i386/udivdi3.S
     i386/umoddi3.S
   )
 
+  if (NOT ANDROID)
+    set(i386_SOURCES
+      ${i386_SOURCES}
+      ${x86_80_BIT_SOURCES}
+      i386/floatdixf.S
+      i386/floatundixf.S
+    )
+  endif()
+
   if (WIN32)
     set(i386_SOURCES
       ${i386_SOURCES}
       i386/chkstk.S
       i386/chkstk2.S
     )
   endif()
 else () # MSVC
   # Use C versions of functions when building on MSVC
   # MSVC's assembler takes Intel syntax, not AT&T syntax.
   # Also use only MSVC compilable builtin implementations.
   set(x86_64_SOURCES
     ${GENERIC_SOURCES}
     ${x86_ARCH_SOURCES}
     x86_64/floatdidf.c
     x86_64/floatdisf.c
-    x86_64/floatdixf.c
   )
   set(i386_SOURCES ${GENERIC_SOURCES} ${x86_ARCH_SOURCES})
 endif () # if (NOT MSVC)
 
 set(arm_SOURCES
   arm/fp_mode.c
   arm/bswapdi2.S
   arm/bswapsi2.S