Index: llvm/lib/Support/APFloat.cpp
===================================================================
--- llvm/lib/Support/APFloat.cpp
+++ llvm/lib/Support/APFloat.cpp
@@ -2242,6 +2242,20 @@
     if (!X86SpecialNan && semantics == &semX87DoubleExtended)
       APInt::tcSetBit(significandParts(), semantics->precision - 1);
 
+    // If we are truncating NaN, it is possible that we shifted out all of the
+    // set bits in a signalling NaN payload. But NaN must remain NaN, so some
+    // bit in the significand must be set (otherwise it is Inf).
+    // Set the 1st bit after the quiet bit to preserve SNaN.
+    // TODO: We should check if the shifting loses any bits above here and set
+    //       losesInfo.
+    // TODO: Would it be better to set quiet and return opInvalidOp?
+    if (APInt::tcIsZero(significandParts(), newPartCount)) {
+      assert(shift < 0 && "Should not lose NaN payload on extend");
+      assert(semantics->precision >= 3 && "Unexpectedly narrow significand");
+      APInt::tcSetBit(significandParts(), semantics->precision - 3);
+      *losesInfo = true;
+    }
+
     // gcc forces the Quiet bit on, which means (float)(double)(float_sNan)
     // does not give you back the same bits.  This is dubious, and we
     // don't currently do it.  You're really supposed to get
Index: llvm/test/Transforms/InstSimplify/ConstProp/cast.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/ConstProp/cast.ll
+++ llvm/test/Transforms/InstSimplify/ConstProp/cast.ll
@@ -39,19 +39,25 @@
   ret float %i
 }
 
-; https://llvm.org/PR43907
+; https://llvm.org/PR43907 - make sure that NaN doesn't morph into Inf.
+; SNaN remains SNaN.
 
 define float @nan_f64_trunc() {
 ; CHECK-LABEL: @nan_f64_trunc(
-; CHECK-NEXT:    ret float 0x7FF0000000000000
+; CHECK-NEXT:    ret float 0x7FF4000000000000
 ;
   %f = fptrunc double 0x7FF0000000000001 to float
   ret float %f
 }
 
+; Verify again with a vector and different destination type.
+; SNaN remains SNaN (first two elements).
+; QNaN remains QNaN (third element).
+; Lower 42 bits of NaN source payload are lost.
+
 define <3 x half> @nan_v3f64_trunc() {
 ; CHECK-LABEL: @nan_v3f64_trunc(
-; CHECK-NEXT:    ret <3 x half> <half 0xH7C00, half 0xH7C00, half 0xH7E00>
+; CHECK-NEXT:    ret <3 x half> <half 0xH7D00, half 0xH7D00, half 0xH7E00>
 ;
   %f = fptrunc <3 x double> <double 0x7FF0020000000000, double 0x7FF003FFFFFFFFFF, double 0x7FF8000000000001> to <3 x half>
   ret <3 x half> %f