[X86/FastIsel] Fall-back to SelectionDAG when lowering soft-floats

[davide]

Summary

FastIsel can't handle them, so we would end up crashing during register class selection.
Fixes https://bugs.llvm.org/show_bug.cgi?id=26522

[MatzeB]

LGTM

[echristo]

OK.

llvm/trunk/lib/Target/X86/X86FastISel.cpp

if (CC != CallingConv::C)
     return false;
 
   if (Subtarget->isCallingConvWin64(CC))
     return false;
 
   if (!Subtarget->is64Bit())
     return false;
 
+  if (Subtarget->useSoftFloat())
+    return false;
+
   // Only handle simple cases. i.e. Up to 6 i32/i64 scalar arguments.
   unsigned GPRCnt = 0;
   unsigned FPRCnt = 0;
   for (auto const &Arg : F->args()) {
     if (Arg.hasAttribute(Attribute::ByVal) ||
         Arg.hasAttribute(Attribute::InReg) ||
         Arg.hasAttribute(Attribute::StructRet) ||
         Arg.hasAttribute(Attribute::SwiftSelf) ||

llvm/trunk/test/CodeGen/X86/fastisel-softfloat.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc %s -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define float @pr26522(float %pat) #0 {
+; CHECK-LABEL: pr26522:
+; CHECK:       # BB#0:
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    retq
+  ret float %pat
+}
+
+attributes #0 = { noinline optnone "target-features"="+soft-float" }