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

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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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228911 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Ahmed Bougacha 2015-02-12 06:15:29 +00:00
parent c571d114ba
commit 9e9bde9b54
3 changed files with 51 additions and 10 deletions
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15
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -7890,11 +7890,16 @@ SDValue DAGCombiner::visitFP_ROUND(SDNode *N) {
// 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)

15
test/CodeGen/AArch64/f16-convert.ll
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@ -133,7 +133,8 @@ define void @store0(i16* nocapture %a, float %val) nounwind {
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, i32 %i, float %val) nounwind {
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, i64 %i, float %val) nounwind {
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, float %val) nounwind {
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, float %val) nounwind {
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

31
test/CodeGen/X86/fp-double-rounding.ll Normal file
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@ -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
}