Index: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -8081,6 +8081,15 @@
       return DAG.getSetCC(DL, ResultVT, Abs, Inf,
                           IsInverted ? ISD::SETONE : ISD::SETOEQ);
     }
+
+    if (TestMask == fcPosInf || TestMask == fcNegInf) {
+      // isposinf(x) --> x == inf
+      // isneginf(x) --> x == -inf
+      SDValue Inf = DAG.getConstantFP(
+        APFloat::getInf(Semantics, TestMask == fcNegInf), DL, OperandVT);
+      return DAG.getSetCC(DL, ResultVT, Op, Inf,
+                          IsInverted ? ISD::SETONE : ISD::SETOEQ);
+    }
   }
 
   // FIXME: Placeholder until FPClassTest is marked as a BitmaskEnum.
Index: llvm/test/CodeGen/X86/is_fpclass-fp80.ll
===================================================================
--- llvm/test/CodeGen/X86/is_fpclass-fp80.ll
+++ llvm/test/CodeGen/X86/is_fpclass-fp80.ll
@@ -282,23 +282,24 @@
 define i1 @is_posinf_f80(x86_fp80 %x) {
 ; CHECK-32-LABEL: is_posinf_f80:
 ; CHECK-32:       # %bb.0: # %entry
-; CHECK-32-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
-; CHECK-32-NEXT:    movl $-2147483648, %ecx # imm = 0x80000000
-; CHECK-32-NEXT:    xorl {{[0-9]+}}(%esp), %ecx
-; CHECK-32-NEXT:    xorl $32767, %eax # imm = 0x7FFF
-; CHECK-32-NEXT:    orl {{[0-9]+}}(%esp), %eax
-; CHECK-32-NEXT:    orl %ecx, %eax
-; CHECK-32-NEXT:    sete %al
+; CHECK-32-NEXT:    fldt {{[0-9]+}}(%esp)
+; CHECK-32-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}
+; CHECK-32-NEXT:    fxch %st(1)
+; CHECK-32-NEXT:    fucompp
+; CHECK-32-NEXT:    fnstsw %ax
+; CHECK-32-NEXT:    # kill: def $ah killed $ah killed $ax
+; CHECK-32-NEXT:    sahf
+; CHECK-32-NEXT:    setae %al
 ; CHECK-32-NEXT:    retl
 ;
 ; CHECK-64-LABEL: is_posinf_f80:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    movzwl {{[0-9]+}}(%rsp), %eax
-; CHECK-64-NEXT:    movabsq $-9223372036854775808, %rcx # imm = 0x8000000000000000
-; CHECK-64-NEXT:    xorq {{[0-9]+}}(%rsp), %rcx
-; CHECK-64-NEXT:    xorq $32767, %rax # imm = 0x7FFF
-; CHECK-64-NEXT:    orq %rcx, %rax
-; CHECK-64-NEXT:    sete %al
+; CHECK-64-NEXT:    fldt {{[0-9]+}}(%rsp)
+; CHECK-64-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}(%rip)
+; CHECK-64-NEXT:    fxch %st(1)
+; CHECK-64-NEXT:    fucompi %st(1), %st
+; CHECK-64-NEXT:    fstp %st(0)
+; CHECK-64-NEXT:    setae %al
 ; CHECK-64-NEXT:    retq
 entry:
   %0 = tail call i1 @llvm.is.fpclass.f80(x86_fp80 %x, i32 512)  ; 0x200 = "+inf"
@@ -308,24 +309,22 @@
 define i1 @is_neginf_f80(x86_fp80 %x) {
 ; CHECK-32-LABEL: is_neginf_f80:
 ; CHECK-32:       # %bb.0: # %entry
-; CHECK-32-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
-; CHECK-32-NEXT:    xorl $65535, %eax # imm = 0xFFFF
-; CHECK-32-NEXT:    movl $-2147483648, %ecx # imm = 0x80000000
-; CHECK-32-NEXT:    xorl {{[0-9]+}}(%esp), %ecx
-; CHECK-32-NEXT:    orl {{[0-9]+}}(%esp), %eax
-; CHECK-32-NEXT:    orl %ecx, %eax
-; CHECK-32-NEXT:    sete %al
+; CHECK-32-NEXT:    fldt {{[0-9]+}}(%esp)
+; CHECK-32-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}
+; CHECK-32-NEXT:    fucompp
+; CHECK-32-NEXT:    fnstsw %ax
+; CHECK-32-NEXT:    # kill: def $ah killed $ah killed $ax
+; CHECK-32-NEXT:    sahf
+; CHECK-32-NEXT:    setae %al
 ; CHECK-32-NEXT:    retl
 ;
 ; CHECK-64-LABEL: is_neginf_f80:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    movl {{[0-9]+}}(%rsp), %eax
-; CHECK-64-NEXT:    notl %eax
-; CHECK-64-NEXT:    movzwl %ax, %eax
-; CHECK-64-NEXT:    movabsq $-9223372036854775808, %rcx # imm = 0x8000000000000000
-; CHECK-64-NEXT:    xorq {{[0-9]+}}(%rsp), %rcx
-; CHECK-64-NEXT:    orq %rax, %rcx
-; CHECK-64-NEXT:    sete %al
+; CHECK-64-NEXT:    fldt {{[0-9]+}}(%rsp)
+; CHECK-64-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}(%rip)
+; CHECK-64-NEXT:    fucompi %st(1), %st
+; CHECK-64-NEXT:    fstp %st(0)
+; CHECK-64-NEXT:    setae %al
 ; CHECK-64-NEXT:    retq
 entry:
   %0 = tail call i1 @llvm.is.fpclass.f80(x86_fp80 %x, i32 4)  ; "-inf"
Index: llvm/test/CodeGen/X86/is_fpclass.ll
===================================================================
--- llvm/test/CodeGen/X86/is_fpclass.ll
+++ llvm/test/CodeGen/X86/is_fpclass.ll
@@ -193,15 +193,20 @@
 define i1 @is_plus_inf_f(float %x) {
 ; CHECK-32-LABEL: is_plus_inf_f:
 ; CHECK-32:       # %bb.0: # %entry
-; CHECK-32-NEXT:    cmpl $2139095040, {{[0-9]+}}(%esp) # imm = 0x7F800000
-; CHECK-32-NEXT:    sete %al
+; CHECK-32-NEXT:    flds {{[0-9]+}}(%esp)
+; CHECK-32-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}
+; CHECK-32-NEXT:    fxch %st(1)
+; CHECK-32-NEXT:    fucompp
+; CHECK-32-NEXT:    fnstsw %ax
+; CHECK-32-NEXT:    # kill: def $ah killed $ah killed $ax
+; CHECK-32-NEXT:    sahf
+; CHECK-32-NEXT:    setae %al
 ; CHECK-32-NEXT:    retl
 ;
 ; CHECK-64-LABEL: is_plus_inf_f:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    movd %xmm0, %eax
-; CHECK-64-NEXT:    cmpl $2139095040, %eax # imm = 0x7F800000
-; CHECK-64-NEXT:    sete %al
+; CHECK-64-NEXT:    ucomiss {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
+; CHECK-64-NEXT:    setae %al
 ; CHECK-64-NEXT:    retq
 entry:
   %0 = tail call i1 @llvm.is.fpclass.f32(float %x, i32 512)  ; 0x200 = "+inf"
@@ -211,15 +216,20 @@
 define i1 @is_minus_inf_f(float %x) {
 ; CHECK-32-LABEL: is_minus_inf_f:
 ; CHECK-32:       # %bb.0: # %entry
-; CHECK-32-NEXT:    cmpl $-8388608, {{[0-9]+}}(%esp) # imm = 0xFF800000
-; CHECK-32-NEXT:    sete %al
+; CHECK-32-NEXT:    flds {{[0-9]+}}(%esp)
+; CHECK-32-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}
+; CHECK-32-NEXT:    fucompp
+; CHECK-32-NEXT:    fnstsw %ax
+; CHECK-32-NEXT:    # kill: def $ah killed $ah killed $ax
+; CHECK-32-NEXT:    sahf
+; CHECK-32-NEXT:    setae %al
 ; CHECK-32-NEXT:    retl
 ;
 ; CHECK-64-LABEL: is_minus_inf_f:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    movd %xmm0, %eax
-; CHECK-64-NEXT:    cmpl $-8388608, %eax # imm = 0xFF800000
-; CHECK-64-NEXT:    sete %al
+; CHECK-64-NEXT:    movss {{.*#+}} xmm1 = mem[0],zero,zero,zero
+; CHECK-64-NEXT:    ucomiss %xmm0, %xmm1
+; CHECK-64-NEXT:    setae %al
 ; CHECK-64-NEXT:    retq
 entry:
   %0 = tail call i1 @llvm.is.fpclass.f32(float %x, i32 4)  ; "-inf"
@@ -229,14 +239,18 @@
 define i1 @not_is_minus_inf_f(float %x) {
 ; CHECK-32-LABEL: not_is_minus_inf_f:
 ; CHECK-32:       # %bb.0: # %entry
-; CHECK-32-NEXT:    cmpl $-8388608, {{[0-9]+}}(%esp) # imm = 0xFF800000
+; CHECK-32-NEXT:    flds {{[0-9]+}}(%esp)
+; CHECK-32-NEXT:    flds {{\.?LCPI[0-9]+_[0-9]+}}
+; CHECK-32-NEXT:    fucompp
+; CHECK-32-NEXT:    fnstsw %ax
+; CHECK-32-NEXT:    # kill: def $ah killed $ah killed $ax
+; CHECK-32-NEXT:    sahf
 ; CHECK-32-NEXT:    setne %al
 ; CHECK-32-NEXT:    retl
 ;
 ; CHECK-64-LABEL: not_is_minus_inf_f:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    movd %xmm0, %eax
-; CHECK-64-NEXT:    cmpl $-8388608, %eax # imm = 0xFF800000
+; CHECK-64-NEXT:    ucomiss {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
 ; CHECK-64-NEXT:    setne %al
 ; CHECK-64-NEXT:    retq
 entry: