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[MIPS] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

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MIPS32 has instructions for efficient count-leading/trailing-zeros, so this should be
considered a cheap operation (and therefore fair game for speculation) for any MIPS32
implementation.

The net result of allowing this speculation for the regression tests in this patch is
that we get this code:

ctlz:
  jr  $ra
  clz  $2, $4

cttz:
  addiu  $1, $4, -1
  not  $2, $4
  and  $1, $2, $1
  clz  $1, $1
  addiu  $2, $zero, 32
  jr  $ra
  subu  $2, $2, $1

Instead of:

ctlz:
  beqz  $4, $BB0_2
  addiu  $2, $zero, 32
  clz  $2, $4
$BB0_2:
  jr  $ra
  nop

cttz:
  beqz  $4, $BB1_2
  addiu  $2, $zero, 32
  addiu  $1, $4, -1
  not  $2, $4
  and  $1, $2, $1
  clz  $1, $1
  addiu  $2, $zero, 32
  subu  $2, $2, $1
$BB1_2:
  jr  $ra
  nop

See D14469 for the larger motivation.

Differential Revision: http://reviews.llvm.org/D14500



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252755 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2015-11-11 17:24:56 +00:00
parent e5bd698a3c
commit 03849aaa4e
4 changed files with 59 additions and 0 deletions
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8
lib/Target/Mips/MipsISelLowering.cpp
View File

@ -834,6 +834,14 @@ SDValue MipsTargetLowering::PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI)
return SDValue();
}
bool MipsTargetLowering::isCheapToSpeculateCttz() const {
return Subtarget.hasMips32();
}
bool MipsTargetLowering::isCheapToSpeculateCtlz() const {
return Subtarget.hasMips32();
}
void
MipsTargetLowering::LowerOperationWrapper(SDNode *N,
SmallVectorImpl<SDValue> &Results,

3
lib/Target/Mips/MipsISelLowering.h
View File

@ -235,6 +235,9 @@ namespace llvm {
return MVT::i32;
}
bool isCheapToSpeculateCttz() const override;
bool isCheapToSpeculateCtlz() const override;
void LowerOperationWrapper(SDNode *N,
SmallVectorImpl<SDValue> &Results,
SelectionDAG &DAG) const override;

43
test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll Normal file
View File

@ -0,0 +1,43 @@
; RUN: opt -S -simplifycfg -mtriple=mips-linux-gnu < %s | FileCheck %s
define i32 @ctlz(i32 %A) {
; CHECK-LABEL: @ctlz(
; CHECK: [[ICMP:%[A-Za-z0-9]+]] = icmp eq i32 %A, 0
; CHECK-NEXT: [[CTZ:%[A-Za-z0-9]+]] = tail call i32 @llvm.ctlz.i32(i32 %A, i1 true)
; CHECK-NEXT: [[SEL:%[A-Za-z0-9.]+]] = select i1 [[ICMP]], i32 32, i32 [[CTZ]]
; CHECK-NEXT: ret i32 [[SEL]]
entry:
%tobool = icmp eq i32 %A, 0
br i1 %tobool, label %cond.end, label %cond.true
cond.true:
%0 = tail call i32 @llvm.ctlz.i32(i32 %A, i1 true)
br label %cond.end
cond.end:
%cond = phi i32 [ %0, %cond.true ], [ 32, %entry ]
ret i32 %cond
}
define i32 @cttz(i32 %A) {
; CHECK-LABEL: @cttz(
; CHECK: [[ICMP:%[A-Za-z0-9]+]] = icmp eq i32 %A, 0
; CHECK-NEXT: [[CTZ:%[A-Za-z0-9]+]] = tail call i32 @llvm.cttz.i32(i32 %A, i1 true)
; CHECK-NEXT: [[SEL:%[A-Za-z0-9.]+]] = select i1 [[ICMP]], i32 32, i32 [[CTZ]]
; CHECK-NEXT: ret i32 [[SEL]]
entry:
%tobool = icmp eq i32 %A, 0
br i1 %tobool, label %cond.end, label %cond.true
cond.true:
%0 = tail call i32 @llvm.cttz.i32(i32 %A, i1 true)
br label %cond.end
cond.end:
%cond = phi i32 [ %0, %cond.true ], [ 32, %entry ]
ret i32 %cond
}
declare i32 @llvm.ctlz.i32(i32, i1)
declare i32 @llvm.cttz.i32(i32, i1)

5
test/Transforms/SimplifyCFG/Mips/lit.local.cfg Normal file
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@ -0,0 +1,5 @@
config.suffixes = ['.ll']
targets = set(config.root.targets_to_build.split())
if not 'Mips' in targets:
config.unsupported = True