[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252755 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/Mips/MipsISelLowering.cpp b/lib/Target/Mips/MipsISelLowering.cpp
index 66c495bc3e92fdc4743e2b5a73bb492bfc3f7449..142d14a13a29cbf3f62f8af9b9e45bb07c2c11ed 100644 (file)
--- a/lib/Target/Mips/MipsISelLowering.cpp
+++ b/lib/Target/Mips/MipsISelLowering.cpp
@@ -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,

diff --git a/lib/Target/Mips/MipsISelLowering.h b/lib/Target/Mips/MipsISelLowering.h
index 8e2cb0b4a6c26744db50405667746c5f526609dd..b33e125b81b770e8c0c82c59ea8a3c0c14ce8704 100644 (file)
--- a/lib/Target/Mips/MipsISelLowering.h
+++ b/lib/Target/Mips/MipsISelLowering.h
@@ -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;

diff --git a/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll b/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll
new file mode 100644 (file)
index 0000000..b4bfb51
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/Mips/cttz-ctlz.ll
@@ -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)
+

diff --git a/test/Transforms/SimplifyCFG/Mips/lit.local.cfg b/test/Transforms/SimplifyCFG/Mips/lit.local.cfg
new file mode 100644 (file)
index 0000000..683bfdc
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/Mips/lit.local.cfg
@@ -0,0 +1,5 @@
+config.suffixes = ['.ll']
+
+targets = set(config.root.targets_to_build.split())
+if not 'Mips' in targets:
+    config.unsupported = True