Reland "[X86][BF16] Add libcall for F80 -> BF16 (#109116)" (#109143)

This reverts commit ababfee78714313a0cad87591b819f0944b90d09.

Add X86 FP80 check.

Author: phoebewang

compiler-rt/lib/builtins/CMakeLists.txt

@@ -192,6 +192,7 @@ set(GENERIC_SOURCES
 set(BF16_SOURCES
     extendbfsf2.c
     truncdfbf2.c
+    truncxfbf2.c
     truncsfbf2.c
 )
 

compiler-rt/lib/builtins/fp_trunc.h

@@ -35,6 +35,18 @@ static const int srcSigFracBits = 52;
 // srcBits - srcSigFracBits - 1
 static const int srcExpBits = 11;
 
+#elif defined SRC_80
+typedef xf_float src_t;
+typedef __uint128_t src_rep_t;
+#define SRC_REP_C (__uint128_t)
+// sign bit, exponent and significand occupy the lower 80 bits.
+static const int srcBits = 80;
+static const int srcSigFracBits = 63;
+// -1 accounts for the sign bit.
+// -1 accounts for the explicitly stored integer bit.
+// srcBits - srcSigFracBits - 1 - 1
+static const int srcExpBits = 15;
+
 #elif defined SRC_QUAD
 typedef tf_float src_t;
 typedef __uint128_t src_rep_t;

compiler-rt/lib/builtins/truncxfbf2.c

@@ -0,0 +1,16 @@
+//===-- lib/truncxfbf2.c - long double -> bfloat 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
+//
+//===----------------------------------------------------------------------===//
+
+#if defined(CRT_HAS_TF_MODE) && __LDBL_MANT_DIG__ == 64 && defined(__x86_64__)
+#define SRC_80
+#define DST_BFLOAT
+#include "fp_trunc_impl.inc"
+
+COMPILER_RT_ABI dst_t __truncxfbf2(long double a) { return __truncXfYf2__(a); }
+
+#endif

llvm/include/llvm/IR/RuntimeLibcalls.def

@@ -367,6 +367,7 @@ HANDLE_LIBCALL(FPROUND_F128_F16, "__trunctfhf2")
 HANDLE_LIBCALL(FPROUND_PPCF128_F16, "__trunctfhf2")
 HANDLE_LIBCALL(FPROUND_F32_BF16, "__truncsfbf2")
 HANDLE_LIBCALL(FPROUND_F64_BF16, "__truncdfbf2")
+HANDLE_LIBCALL(FPROUND_F80_BF16, "__truncxfbf2")
 HANDLE_LIBCALL(FPROUND_F64_F32, "__truncdfsf2")
 HANDLE_LIBCALL(FPROUND_F80_F32, "__truncxfsf2")
 HANDLE_LIBCALL(FPROUND_F128_F32, "__trunctfsf2")

llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -169,6 +169,8 @@ RTLIB::Libcall RTLIB::getFPROUND(EVT OpVT, EVT RetVT) {
       return FPROUND_F32_BF16;
     if (OpVT == MVT::f64)
       return FPROUND_F64_BF16;
+    if (OpVT == MVT::f80)
+      return FPROUND_F80_BF16;
   } else if (RetVT == MVT::f32) {
     if (OpVT == MVT::f64)
       return FPROUND_F64_F32;

llvm/test/CodeGen/X86/bfloat.ll

@@ -1953,3 +1953,25 @@ define void @PR92471(ptr %0, ptr %1) nounwind {
   store <7 x float> %4, ptr %1, align 4
   ret void
 }
+
+define bfloat @PR108936(x86_fp80 %0) nounwind {
+; X86-LABEL: PR108936:
+; X86:       # %bb.0:
+; X86-NEXT:    subl $12, %esp
+; X86-NEXT:    fldt {{[0-9]+}}(%esp)
+; X86-NEXT:    fstpt (%esp)
+; X86-NEXT:    calll __truncxfbf2
+; X86-NEXT:    addl $12, %esp
+; X86-NEXT:    retl
+;
+; CHECK-LABEL: PR108936:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    subq $24, %rsp
+; CHECK-NEXT:    fldt {{[0-9]+}}(%rsp)
+; CHECK-NEXT:    fstpt (%rsp)
+; CHECK-NEXT:    callq __truncxfbf2@PLT
+; CHECK-NEXT:    addq $24, %rsp
+; CHECK-NEXT:    retq
+  %2 = fptrunc x86_fp80 %0 to bfloat
+  ret bfloat %2
+}