[LoopIdiomRecognize] Add CTTZ support

Summary:
Existing LIR recognizes CTLZ where shifting input variable right until it is zero. (Shift-Until-Zero idiom)

This commit:
1. Augments Shift-Until-Zero idiom to recognize CTTZ where input variable is shifted left.
2. Prepare for BitScan idiom recognition.

Patch by Yuanfang Chen (tabloid.adroit)

Reviewers: craig.topper, evstupac

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55876

llvm-svn: 350074

1 files changed, 82 insertions, 0 deletions

diff --git a/llvm/test/Transforms/LoopIdiom/X86/cttz.ll b/llvm/test/Transforms/LoopIdiom/X86/cttz.ll
new file mode 100644
index 00000000000..e18fecabe86
--- /dev/null
+++ b/llvm/test/Transforms/LoopIdiom/X86/cttz.ll
@@ -0,0 +1,82 @@
+; RUN: opt -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 < %s -S | FileCheck --check-prefix=ALL %s
+; RUN: opt -loop-idiom -mtriple=x86_64 -mcpu=corei7 < %s -S | FileCheck --check-prefix=ALL %s
+
+; Recognize CTTZ builtin pattern.
+; Here it will replace the loop -
+; assume builtin is always profitable.
+;
+; int cttz_zero_check(int n)
+; {
+;   int i = 0;
+;   while(n) {
+;     n <<= 1;
+;     i++;
+;   }
+;   return i;
+; }
+;
+; ALL-LABEL: @cttz_zero_check
+; ALL:       %0 = call i32 @llvm.cttz.i32(i32 %n, i1 true)
+; ALL-NEXT:  %1 = sub i32 32, %0
+;
+; Function Attrs: norecurse nounwind readnone uwtable
+define i32 @cttz_zero_check(i32 %n) {
+entry:
+  %tobool4 = icmp eq i32 %n, 0
+  br i1 %tobool4, label %while.end, label %while.body.preheader
+
+while.body.preheader:                             ; preds = %entry
+  br label %while.body
+
+while.body:                                       ; preds = %while.body.preheader, %while.body
+  %i.06 = phi i32 [ %inc, %while.body ], [ 0, %while.body.preheader ]
+  %n.addr.05 = phi i32 [ %shl, %while.body ], [ %n, %while.body.preheader ]
+  %shl = shl i32 %n.addr.05, 1
+  %inc = add nsw i32 %i.06, 1
+  %tobool = icmp eq i32 %shl, 0
+  br i1 %tobool, label %while.end.loopexit, label %while.body
+
+while.end.loopexit:                               ; preds = %while.body
+  br label %while.end
+
+while.end:                                        ; preds = %while.end.loopexit, %entry
+  %i.0.lcssa = phi i32 [ 0, %entry ], [ %inc, %while.end.loopexit ]
+  ret i32 %i.0.lcssa
+}
+
+; Recognize CTTZ builtin pattern.
+; Here it will replace the loop -
+; assume builtin is always profitable.
+;
+; int cttz(int n)
+; {
+;   int i = 0;
+;   while(n <<= 1) {
+;     i++;
+;   }
+;   return i;
+; }
+;
+; ALL-LABEL: @cttz
+; ALL:      %0 = shl i32 %n, 1
+; ALL-NEXT: %1 = call i32 @llvm.cttz.i32(i32 %0, i1 false)
+; ALL-NEXT: %2 = sub i32 32, %1
+; ALL-NEXT: %3 = add i32 %2, 1
+;
+; Function Attrs: norecurse nounwind readnone uwtable
+define i32 @cttz(i32 %n) {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %while.cond, %entry
+  %n.addr.0 = phi i32 [ %n, %entry ], [ %shl, %while.cond ]
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %while.cond ]
+  %shl = shl i32 %n.addr.0, 1
+  %tobool = icmp eq i32 %shl, 0
+  %inc = add nsw i32 %i.0, 1
+  br i1 %tobool, label %while.end, label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  ret i32 %i.0
+}
+