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[DAGCombine] Don't combine sext with extload if sextload is not supported and extload has multi users 

In function DAGCombiner::visitSIGN_EXTEND_INREG, sext can be combined with extload even if sextload is not supported by target, then

  if sext is the only user of extload, there is no big difference, no harm no benefit.
  if extload has more than one user, the combined sextload may block extload from combining with other zext, causes extra zext instructions generated. As demonstrated by the attached test case.

This patch add the constraint that when sextload is not supported by target, sext can only be combined with extload if it is the only user of extload.

Differential Revision: https://reviews.llvm.org/D39108

llvm-svn: 316802
This commit is contained in:
Guozhi Wei 2017-10-27 21:54:24 +00:00
parent de370414e3
commit 7c67009fe5
2 changed files with 31 additions and 1 deletions
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6
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -8274,10 +8274,14 @@ SDValue DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
}
// fold (sext_inreg (extload x)) -> (sextload x)
// If sextload is not supported by target, we can only do the combine when
// load has one use. Doing otherwise can block folding the extload with other
// extends that the target does support.
if (ISD::isEXTLoad(N0.getNode()) &&
ISD::isUNINDEXEDLoad(N0.getNode()) &&
EVT == cast<LoadSDNode>(N0)->getMemoryVT() &&
((!LegalOperations && !cast<LoadSDNode>(N0)->isVolatile()) ||
((!LegalOperations && !cast<LoadSDNode>(N0)->isVolatile() &&
N0.hasOneUse()) ||
TLI.isLoadExtLegal(ISD::SEXTLOAD, VT, EVT))) {
LoadSDNode *LN0 = cast<LoadSDNode>(N0);
SDValue ExtLoad = DAG.getExtLoad(ISD::SEXTLOAD, SDLoc(N), VT,

26
llvm/test/CodeGen/PowerPC/selectiondag-sextload.ll Normal file
View File

@ -0,0 +1,26 @@
; RUN: llc --mtriple=powerpc64le-linux-gnu < %s | FileCheck %s
; It tests in function DAGCombiner::visitSIGN_EXTEND_INREG
; signext will not be combined with extload, and causes extra zext.
declare void @g(i32 signext)
define void @foo(i8* %p) {
entry:
br label %while.body
while.body:
%0 = load i8, i8* %p, align 1
%conv = zext i8 %0 to i32
%cmp = icmp sgt i8 %0, 0
br i1 %cmp, label %if.then, label %while.body
; CHECK: lbz
; CHECK: extsb.
; CHECK-NOT: rlwinm
; CHECK: ble
if.then:
tail call void @g(i32 signext %conv)
br label %while.body
}