[llvm] r293713 - [NVPTX] Compute approx sqrt as 1/rsqrt(x) rather than x*rsqrt(x).

[Justin Lebar via llvm-commits]

Author: jlebar
Date: Tue Jan 31 17:08:57 2017
New Revision: 293713

URL: http://llvm.org/viewvc/llvm-project?rev=293713&view=rev
Log:
[NVPTX] Compute approx sqrt as 1/rsqrt(x) rather than x*rsqrt(x).

x*rsqrt(x) returns NaN for x == 0, whereas 1/rsqrt(x) returns 0, as
desired.

Verified that the particular nvptx approximate instructions here do in
fact return 0 for x = 0.

Modified:
    llvm/trunk/lib/Target/NVPTX/NVPTXISelLowering.cpp
    llvm/trunk/test/CodeGen/NVPTX/fast-math.ll
    llvm/trunk/test/CodeGen/NVPTX/sqrt-approx.ll

Modified: llvm/trunk/lib/Target/NVPTX/NVPTXISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/NVPTX/NVPTXISelLowering.cpp?rev=293713&r1=293712&r2=293713&view=diff
==============================================================================
--- llvm/trunk/lib/Target/NVPTX/NVPTXISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/NVPTX/NVPTXISelLowering.cpp Tue Jan 31 17:08:57 2017
@@ -1080,9 +1080,14 @@ SDValue NVPTXTargetLowering::getSqrtEsti
       return MakeIntrinsicCall(Ftz ? Intrinsic::nvvm_sqrt_approx_ftz_f
                                    : Intrinsic::nvvm_sqrt_approx_f);
     else {
-      // There's no sqrt.approx.f64 instruction, so we emit x * rsqrt(x).
-      return DAG.getNode(ISD::FMUL, DL, VT, Operand,
-                         MakeIntrinsicCall(Intrinsic::nvvm_rsqrt_approx_d));
+      // There's no sqrt.approx.f64 instruction, so we emit
+      // reciprocal(rsqrt(x)).  This is faster than
+      // select(x == 0, 0, x * rsqrt(x)).  (In fact, it's faster than plain
+      // x * rsqrt(x).)
+      return DAG.getNode(
+          ISD::INTRINSIC_WO_CHAIN, DL, VT,
+          DAG.getConstant(Intrinsic::nvvm_rcp_approx_ftz_d, DL, MVT::i32),
+          MakeIntrinsicCall(Intrinsic::nvvm_rsqrt_approx_d));
     }
   }
 }

Modified: llvm/trunk/test/CodeGen/NVPTX/fast-math.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/NVPTX/fast-math.ll?rev=293713&r1=293712&r2=293713&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/NVPTX/fast-math.ll (original)
+++ llvm/trunk/test/CodeGen/NVPTX/fast-math.ll Tue Jan 31 17:08:57 2017
@@ -40,11 +40,11 @@ define float @sqrt_div_fast_ftz(float %a
 }
 
 ; There are no fast-math or ftz versions of sqrt and div for f64.  We use
-; x * rsqrt(x) for sqrt(x), and emit a vanilla divide.
+; reciprocal(rsqrt(x)) for sqrt(x), and emit a vanilla divide.
 
 ; CHECK-LABEL: sqrt_div_fast_ftz_f64(
 ; CHECK: rsqrt.approx.f64
-; CHECK: mul.f64
+; CHECK: rcp.approx.ftz.f64
 ; CHECK: div.rn.f64
 define double @sqrt_div_fast_ftz_f64(double %a, double %b) #0 #1 {
   %t1 = tail call double @llvm.sqrt.f64(double %a)

Modified: llvm/trunk/test/CodeGen/NVPTX/sqrt-approx.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/NVPTX/sqrt-approx.ll?rev=293713&r1=293712&r2=293713&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/NVPTX/sqrt-approx.ll (original)
+++ llvm/trunk/test/CodeGen/NVPTX/sqrt-approx.ll Tue Jan 31 17:08:57 2017
@@ -59,9 +59,11 @@ define float @test_sqrt_ftz(float %a) #0
 
 ; CHECK-LABEL test_sqrt64
 define double @test_sqrt64(double %a) #0 {
-; There's no sqrt.approx.f64 instruction; we emit x * rsqrt.approx.f64(x).
+; There's no sqrt.approx.f64 instruction; we emit
+; reciprocal(rsqrt.approx.f64(x)).  There's no non-ftz approximate reciprocal,
+; so we just use the ftz version.
 ; CHECK: rsqrt.approx.f64
-; CHECK: mul.f64
+; CHECK: rcp.approx.ftz.f64
   %ret = tail call double @llvm.sqrt.f64(double %a)
   ret double %ret
 }
@@ -70,7 +72,7 @@ define double @test_sqrt64(double %a) #0
 define double @test_sqrt64_ftz(double %a) #0 #1 {
 ; There's no sqrt.approx.ftz.f64 instruction; we just use the non-ftz version.
 ; CHECK: rsqrt.approx.f64
-; CHECK: mul.f64
+; CHECK: rcp.approx.ftz.f64
   %ret = tail call double @llvm.sqrt.f64(double %a)
   ret double %ret
 }