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Don't combine fp_round (fp_round x) if f80 to f16 is generated 

Summary:
This patch skips DAG combine of fp_round (fp_round x) if it results in
an fp_round from f80 to f16.

fp_round from f80 to f16 always generates an expensive (and as yet,
unimplemented) libcall to __truncxfhf2.  This prevents selection of
native f16 conversion instructions from f32 or f64.  Moreover, the first
(value-preserving) fp_round from f80 to either f32 or f64 may become a
NOP in platforms like x86.

Reviewers: ab

Subscribers: srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D17221

llvm-svn: 260769
This commit is contained in:
Pirama Arumuga Nainar 2016-02-13 00:08:05 +00:00
parent 2a8fa2a888
commit 7476bc89e9
2 changed files with 26 additions and 0 deletions
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11
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -9019,6 +9019,17 @@ SDValue DAGCombiner::visitFP_ROUND(SDNode *N) {
if (N0.getOpcode() == ISD::FP_ROUND) {
const bool NIsTrunc = N->getConstantOperandVal(1) == 1;
const bool N0IsTrunc = N0.getNode()->getConstantOperandVal(1) == 1;
// Skip this folding if it results in an fp_round from f80 to f16.
//
// f80 to f16 always generates an expensive (and as yet, unimplemented)
// libcall to __truncxfhf2 instead of selecting native f16 conversion
// instructions from f32 or f64. Moreover, the first (value-preserving)
// fp_round from f80 to either f32 or f64 may become a NOP in platforms like
// x86.
if (N0.getOperand(0).getValueType() == MVT::f80 && VT == MVT::f16)
return SDValue();
// 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.

15
llvm/test/CodeGen/X86/half.ll
View File

@ -2,6 +2,8 @@
; RUN: | FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-LIBCALL
; RUN: llc < %s -march=x86-64 -mtriple=x86_64-unknown-linux-gnu -mcpu=corei7 -mattr=+f16c -asm-verbose=false \
; RUN: | FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-F16C
; RUN: llc < %s -mtriple=i686-unknown-linux-gnu -mattr +sse2 -asm-verbose=false \
; RUN: | FileCheck %s -check-prefix=CHECK-I686
define void @test_load_store(half* %in, half* %out) {
; CHECK-LABEL: test_load_store:
@ -260,4 +262,17 @@ define void @test_trunc64_vec4(<4 x double> %a, <4 x half>* %p) {
ret void
}
declare float @test_floatret();
; On i686, if SSE2 is available, the return value from test_floatret is loaded
; to f80 and then rounded to f32. The DAG combiner should not combine this
; fp_round and the subsequent fptrunc from float to half.
define half @test_f80trunc_nodagcombine() #0 {
; CHECK-LABEL: test_f80trunc_nodagcombine:
; CHECK-I686-NOT: calll __truncxfhf2
%1 = call float @test_floatret()
%2 = fptrunc float %1 to half
ret half %2
}
attributes #0 = { nounwind }