[llvm] r252755 - [MIPS] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Wed Nov 11 11:24:56 2015
New Revision: 252755

URL: http://llvm.org/viewvc/llvm-project?rev=252755&view=rev
Log:
[MIPS] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

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


Added:
    llvm/trunk/test/Transforms/SimplifyCFG/Mips/
    llvm/trunk/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll
    llvm/trunk/test/Transforms/SimplifyCFG/Mips/lit.local.cfg
Modified:
    llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp
    llvm/trunk/lib/Target/Mips/MipsISelLowering.h

Modified: llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp?rev=252755&r1=252754&r2=252755&view=diff
==============================================================================
--- llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp Wed Nov 11 11:24:56 2015
@@ -834,6 +834,14 @@ SDValue  MipsTargetLowering::PerformDAGC
   return SDValue();
 }
 
+bool MipsTargetLowering::isCheapToSpeculateCttz() const {
+  return Subtarget.hasMips32();
+}
+
+bool MipsTargetLowering::isCheapToSpeculateCtlz() const {
+  return Subtarget.hasMips32();
+}
+
 void
 MipsTargetLowering::LowerOperationWrapper(SDNode *N,
                                           SmallVectorImpl<SDValue> &Results,

Modified: llvm/trunk/lib/Target/Mips/MipsISelLowering.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/Mips/MipsISelLowering.h?rev=252755&r1=252754&r2=252755&view=diff
==============================================================================
--- llvm/trunk/lib/Target/Mips/MipsISelLowering.h (original)
+++ llvm/trunk/lib/Target/Mips/MipsISelLowering.h Wed Nov 11 11:24:56 2015
@@ -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;

Added: llvm/trunk/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll?rev=252755&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll (added)
+++ llvm/trunk/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll Wed Nov 11 11:24:56 2015
@@ -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)
+

Added: llvm/trunk/test/Transforms/SimplifyCFG/Mips/lit.local.cfg
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/Mips/lit.local.cfg?rev=252755&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/Mips/lit.local.cfg (added)
+++ llvm/trunk/test/Transforms/SimplifyCFG/Mips/lit.local.cfg Wed Nov 11 11:24:56 2015
@@ -0,0 +1,5 @@
+config.suffixes = ['.ll']
+
+targets = set(config.root.targets_to_build.split())
+if not 'Mips' in targets:
+    config.unsupported = True