[builtins] Build float128 soft float builtins for x86_64.

[starsid]

Summary

float128 builtins are currently not built for x86_64.
This causes linker to complain baout missing symbols when linking
glibc 2.27 with float128 support.

[manojgupta]

lib/builtins/fp_lib.h

107 ↗

(On Diff #170743)

I really don't know the accurate set of checks that should be used here, Please advise,

111 ↗

(On Diff #170743)

Changed long double to __float128 on Eli's advice in PR39376.

[efriedma]

lib/builtins/fp_lib.h

111 ↗

(On Diff #170743)

You need to guard this with an ifdef; clang doesn't supports __float128 on every target, even if long double is an 128-bit IEEE float.

For reasons I don't really understand, there are apparently two different macros for this; #if defined(__FLOAT128__) || defined(__SIZEOF_FLOAT128__) should do the right thing.

[manojgupta]

Added checked for defined(FLOAT128) || defined(SIZEOF_FLOAT128)

[manojgupta]

lib/builtins/fp_lib.h

111 ↗

(On Diff #170743)

Thanks Eli. I also found out that GCC 4.9 does not seem to have these defined even though it supports __float128.
https://godbolt.org/z/ReVm8j

[manojgupta]

I am also getting these tests failures because of missing trunctfxf2 and extendxftf2. These are provided by libgcc but compiler-rt does not seem to have an implementation for them.

Builtins-x86_64-linux :: compiler_rt_logbl_test.c
Builtins-x86_64-linux :: divtc3_test.c
Builtins-x86_64-linux :: floattitf_test.c
Builtins-x86_64-linux :: floatuntitf_test.c

compiler_rt_logbl_test.c:26: error: undefined reference to 'trunctfxf2'
compiler_rt_logbl_test.c:26: error: undefined reference to 'extendxftf2'

[efriedma]

__extendxftf2 and __trunctfxf2 are for conversions between x86 long double and __float128; you'll need to write implementations yourself. (That should be a separate patch.)

Thanks Eli. I also found out that GCC 4.9 does not seem to have these defined even though it supports __float128.

gcc 5 has the define; that's good enough.

Missing changes to run the unittests (test/builtins/Unit/) for the new functions. Unfortunately, it looks like that will be a large patch because they use "long double" and "LDBL_MANT_DIG" explicitly all over the place.

lib/builtins/fp_lib.h

107 ↗

(On Diff #170756)

Instead of checking for __x86_64__, this should probably also check for the FLOAT128 defines.

[manojgupta]

Took another look and seems like long double is hardcoded in many of the builtins. So I think the current patch needs to rename a lot of places using long double to __float128 type.

Some examples where I think __float128 type (propagating the type in fp_lib.h) should be used instead of long double:

./extenddftf2.c: COMPILER_RT_ABI long double extenddftf2(double a) {
./trunctfsf2.c: COMPILER_RT_ABI float trunctfsf2(long double a) {
./extendsftf2.c: COMPILER_RT_ABI long double __extendsftf2(float a) {

@efriedma what do you think?

[efriedma]

Yes, that looks right.

[LuoYuanke]

Hi
What's the status for __float128 support? Has it already been finished?

[manojgupta]

In D53608#1423692, @LuoYuanke wrote:

Hi
What's the status for __float128 support? Has it already been finished?

Sorry, haven't looked at time for a while. Consider this change abandoned for now :(

[troyj]

Wound up here while trying to use compiler-rt for static linking with our downstream compiler. It seems that compiler-rt's current approach of only providing these routines on certain platforms is problematic because libgcc always provides them, and thus compiler-rt is not a full replacement for libgcc in some cases. I also encountered the same two missing entry points.

Would very much like to see an upstream change to match the libgcc entry points, no matter the platform.

[efriedma]

Yes, it makes sense to provide these routines, but someone has to write the code to make it work. This patch is currently incomplete:

Took another look and seems like long double is hardcoded in many of the builtins. So I think the current patch needs to rename a lot of places using long double to __float128 type.

[starsid]

rebase and resurrect

[starsid]

fix commit author name and email

[starsid]

I have tried my best to follow the conversation here and complete this patch. Please let me know if it is missing something, or if I need to assign new reviewers.

[efriedma]

From earlier review comment:

Missing changes to run the unittests (test/builtins/Unit/) for the new functions.

That's the most important part, so we have some confidence the code actually works.

[starsid]

Thanks. I missed that in the comment thread. Will send back for review once ready.

[aaronmondal]

Hmm I tried to fiddle around with this but I'm out of my depth here. Pulling in @MaskRay @compnerd @phosek in the hopes that one of you is able to provide some insight or pull in someone else who is able to help.

When building a static compiler-rt with thinlto every downstream target breaks because of the missing __extendxftf2 and __trunctfxf2. AFAIU this blocks everyone from using thinlto with compiler-rt.

ld.lld: error: undefined symbol: __extendxftf2
>>> referenced by extendsftf2.c:18 (./external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/compiler-rt/lib/builtins/extendsftf2.c:18)
>>>               lto.tmp:(__extendsftf2)
>>> referenced by extenddftf2.c:18 (./external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/compiler-rt/lib/builtins/extenddftf2.c:18)
>>>               lto.tmp:(__extenddftf2)
>>> referenced by floattitf.c:75 (./external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/compiler-rt/lib/builtins/floattitf.c:75)
>>>               lto.tmp:(__floattitf)
>>> referenced 1 more times
>>> did you mean: __extenddftf2
>>> defined in: lto.tmp

ld.lld: error: undefined symbol: __trunctfxf2
>>> referenced by trunctfsf2.c:17 (./external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/compiler-rt/lib/builtins/trunctfsf2.c:17)
>>>               lto.tmp:(__trunctfsf2)
>>> referenced by trunctfdf2.c:17 (./external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/compiler-rt/lib/builtins/trunctfdf2.c:17)
>>>               lto.tmp:(__trunctfdf2)
bazel-out/k8-fastbuild-ST-1b2103630309/bin/external/llvm-project-overlay~17-init-bcr.0~llvm_project_overlay~llvm-project/clang/clang-linker-wrapper: error: 'ld.lld' failed

I also vaguely remember this leading to issues with static CUDA linking, but I don't have stacktraces for that :/

compiler-rt/lib/builtins/extendhftf2.c

 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT) &&                     \
     defined(COMPILER_RT_HAS_FLOAT16)
 #define SRC_HALF
 #define DST_QUAD
 #include "fp_extend_impl.inc"
 
-COMPILER_RT_ABI long double __extendhftf2(_Float16 a) {
-  return __extendXfYf2__(a);
-}
+COMPILER_RT_ABI fp_t __extendhftf2(_Float16 a) { return __extendXfYf2__(a); }
 
 #endif

compiler-rt/lib/builtins/fp_lib.h

static __inline void wideMultiply(rep_t a, rep_t b, rep_t *hi, rep_t *lo) {
   *hi = hiWord(plohi) + hiWord(philo) + hiWord(r1) + phihi;
 }
 #undef loWord
 #undef hiWord
 
 COMPILER_RT_ABI fp_t __adddf3(fp_t a, fp_t b);
 
 #elif defined QUAD_PRECISION
-#if __LDBL_MANT_DIG__ == 113 && defined(__SIZEOF_INT128__)
+#if (__LDBL_MANT_DIG__ == 113 || defined(__x86_64__)) &&                       \
+    defined(__SIZEOF_INT128__)
 #define CRT_LDBL_128BIT
 typedef uint64_t half_rep_t;
 typedef __uint128_t rep_t;
 typedef __int128_t srep_t;
+#if defined(__FLOAT128__) || defined(__SIZEOF_FLOAT128__)
+typedef __float128 fp_t;
+#else
 typedef long double fp_t;
+#endif
 #define HALF_REP_C UINT64_C
 #define REP_C (__uint128_t)
 // Note: Since there is no explicit way to tell compiler the constant is a
 // 128-bit integer, we let the constant be casted to 128-bit integer
 #define significandBits 112
 
 static __inline int rep_clz(rep_t a) {
   const union {

compiler-rt/lib/builtins/powitf2.c

 
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
 
 // Returns: a ^ b
 
-COMPILER_RT_ABI long double __powitf2(long double a, int b) {
+COMPILER_RT_ABI fp_t __powitf2(fp_t a, int b) {
   const int recip = b < 0;
-  long double r = 1;
+  fp_t r = 1;
   while (1) {
     if (b & 1)
       r *= a;
     b /= 2;
     if (b == 0)
       break;
     a *= a;
   }
   return recip ? 1 / r : r;
 }
 
 #endif

compiler-rt/lib/builtins/trunctfdf2.c

 //===-- lib/truncdfsf2.c - quad -> double conversion --------------*- C -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
 #define SRC_QUAD
 #define DST_DOUBLE
 #include "fp_trunc_impl.inc"
 
-COMPILER_RT_ABI double __trunctfdf2(long double a) { return __truncXfYf2__(a); }
+COMPILER_RT_ABI double __trunctfdf2(fp_t a) { return __truncXfYf2__(a); }
 
 #endif

compiler-rt/lib/builtins/trunctfhf2.c

 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT) &&                     \
     defined(COMPILER_RT_HAS_FLOAT16)
 #define SRC_QUAD
 #define DST_HALF
 #include "fp_trunc_impl.inc"
 
-COMPILER_RT_ABI _Float16 __trunctfhf2(long double a) {
-  return __truncXfYf2__(a);
-}
+COMPILER_RT_ABI _Float16 __trunctfhf2(fp_t a) { return __truncXfYf2__(a); }
 
 #endif

compiler-rt/lib/builtins/trunctfsf2.c

 //===-- lib/trunctfsf2.c - quad -> single conversion --------------*- C -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
 #define SRC_QUAD
 #define DST_SINGLE
 #include "fp_trunc_impl.inc"
 
-COMPILER_RT_ABI float __trunctfsf2(long double a) { return __truncXfYf2__(a); }
+COMPILER_RT_ABI float __trunctfsf2(fp_t a) { return __truncXfYf2__(a); }
 
 #endif