[llvm] r228911 - [CodeGen] Don't blindly combine (fp_round (fp_round x)) to (fp_round x).

[Ahmed Bougacha]

Author: ab
Date: Thu Feb 12 00:15:29 2015
New Revision: 228911

URL: http://llvm.org/viewvc/llvm-project?rev=228911&view=rev
Log:
[CodeGen] Don't blindly combine (fp_round (fp_round x)) to (fp_round x).

We used to do this DAG combine, but it's not always correct:
If the first fp_round isn't a value preserving truncation, it might
introduce a tie in the second fp_round, that wouldn't occur in the
single-step fp_round we want to fold to.
In other words, double rounding isn't the same as rounding.

Differential Revision: http://reviews.llvm.org/D7571

Added:
    llvm/trunk/test/CodeGen/X86/fp-double-rounding.ll
Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/trunk/test/CodeGen/AArch64/f16-convert.ll

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=228911&r1=228910&r2=228911&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Thu Feb 12 00:15:29 2015
@@ -7890,11 +7890,16 @@ SDValue DAGCombiner::visitFP_ROUND(SDNod
 
   // fold (fp_round (fp_round x)) -> (fp_round x)
   if (N0.getOpcode() == ISD::FP_ROUND) {
-    // This is a value preserving truncation if both round's are.
-    bool IsTrunc = N->getConstantOperandVal(1) == 1 &&
-                   N0.getNode()->getConstantOperandVal(1) == 1;
-    return DAG.getNode(ISD::FP_ROUND, SDLoc(N), VT, N0.getOperand(0),
-                       DAG.getIntPtrConstant(IsTrunc));
+    const bool NIsTrunc = N->getConstantOperandVal(1) == 1;
+    const bool N0IsTrunc = N0.getNode()->getConstantOperandVal(1) == 1;
+    // If the first fp_round isn't a value preserving truncation, it might
+    // introduce a tie in the second fp_round, that wouldn't occur in the
+    // single-step fp_round we want to fold to.
+    // In other words, double rounding isn't the same as rounding.
+    // Also, this is a value preserving truncation iff both fp_round's are.
+    if (DAG.getTarget().Options.UnsafeFPMath || N0IsTrunc)
+      return DAG.getNode(ISD::FP_ROUND, SDLoc(N), VT, N0.getOperand(0),
+                         DAG.getIntPtrConstant(NIsTrunc && N0IsTrunc));
   }
 
   // fold (fp_round (copysign X, Y)) -> (copysign (fp_round X), Y)

Modified: llvm/trunk/test/CodeGen/AArch64/f16-convert.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/f16-convert.ll?rev=228911&r1=228910&r2=228911&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/f16-convert.ll (original)
+++ llvm/trunk/test/CodeGen/AArch64/f16-convert.ll Thu Feb 12 00:15:29 2015
@@ -133,7 +133,8 @@ define void @store0(i16* nocapture %a, f
 
 define void @store1(i16* nocapture %a, double %val) nounwind {
 ; CHECK-LABEL: store1:
-; CHECK-NEXT: fcvt h0, d0
+; CHECK-NEXT: fcvt s0, d0
+; CHECK-NEXT: fcvt h0, s0
 ; CHECK-NEXT: str  h0, [x0]
 ; CHECK-NEXT: ret
 
@@ -158,7 +159,8 @@ define void @store2(i16* nocapture %a, i
 
 define void @store3(i16* nocapture %a, i32 %i, double %val) nounwind {
 ; CHECK-LABEL: store3:
-; CHECK-NEXT: fcvt h0, d0
+; CHECK-NEXT: fcvt s0, d0
+; CHECK-NEXT: fcvt h0, s0
 ; CHECK-NEXT: str h0, [x0, w1, sxtw #1]
 ; CHECK-NEXT: ret
 
@@ -184,7 +186,8 @@ define void @store4(i16* nocapture %a, i
 
 define void @store5(i16* nocapture %a, i64 %i, double %val) nounwind {
 ; CHECK-LABEL: store5:
-; CHECK-NEXT: fcvt h0, d0
+; CHECK-NEXT: fcvt s0, d0
+; CHECK-NEXT: fcvt h0, s0
 ; CHECK-NEXT: str h0, [x0, x1, lsl #1]
 ; CHECK-NEXT: ret
 
@@ -209,7 +212,8 @@ define void @store6(i16* nocapture %a, f
 
 define void @store7(i16* nocapture %a, double %val) nounwind {
 ; CHECK-LABEL: store7:
-; CHECK-NEXT: fcvt h0, d0
+; CHECK-NEXT: fcvt s0, d0
+; CHECK-NEXT: fcvt h0, s0
 ; CHECK-NEXT: str h0, [x0, #20]
 ; CHECK-NEXT: ret
 
@@ -234,7 +238,8 @@ define void @store8(i16* nocapture %a, f
 
 define void @store9(i16* nocapture %a, double %val) nounwind {
 ; CHECK-LABEL: store9:
-; CHECK-NEXT: fcvt h0, d0
+; CHECK-NEXT: fcvt s0, d0
+; CHECK-NEXT: fcvt h0, s0
 ; CHECK-NEXT: stur h0, [x0, #-20]
 ; CHECK-NEXT: ret
 

Added: llvm/trunk/test/CodeGen/X86/fp-double-rounding.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/fp-double-rounding.ll?rev=228911&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/X86/fp-double-rounding.ll (added)
+++ llvm/trunk/test/CodeGen/X86/fp-double-rounding.ll Thu Feb 12 00:15:29 2015
@@ -0,0 +1,31 @@
+; RUN: llc < %s | FileCheck %s --check-prefix=CHECK --check-prefix=SAFE
+; RUN: llc < %s -enable-unsafe-fp-math | FileCheck %s --check-prefix=CHECK --check-prefix=UNSAFE
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64--"
+
+; CHECK-LABEL: double_rounding:
+; SAFE: callq __trunctfdf2
+; SAFE-NEXT: cvtsd2ss %xmm0
+; UNSAFE: callq __trunctfsf2
+; UNSAFE-NOT: cvt
+define void @double_rounding(fp128* %x, float* %f) {
+entry:
+  %0 = load fp128* %x, align 16
+  %1 = fptrunc fp128 %0 to double
+  %2 = fptrunc double %1 to float
+  store float %2, float* %f, align 4
+  ret void
+}
+
+; CHECK-LABEL: double_rounding_precise_first:
+; CHECK: fstps (%
+; CHECK-NOT: fstpl
+define void @double_rounding_precise_first(float* %f) {
+entry:
+  ; Hack, to generate a precise FP_ROUND to double
+  %precise = call double asm sideeffect "fld %st(0)", "={st(0)}"()
+  %0 = fptrunc double %precise to float
+  store float %0, float* %f, align 4
+  ret void
+}