[BDCE] SExt -> ZExt when no sign bits is used and instruction has mul…

…tiple uses

Summary: This allows to convert any SExt to a ZExt when we know none of the extended bits are used, specially in cases where there are multiple uses of the value.

Reviewers: dmgreen, eli.friedman, spatel, lebedev.ri, nikic

Reviewed By: lebedev.ri, nikic

Subscribers: hiraditya, dmgreen, craig.topper, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60413

llvm/lib/Transforms/Scalar/BDCE.cpp

@@ -9,7 +9,8 @@
 // This file implements the Bit-Tracking Dead Code Elimination pass. Some
 // instructions (shifts, some ands, ors, etc.) kill some of their input bits.
 // We track these dead bits and remove instructions that compute only these
-// dead bits.
+// dead bits. We also simplify sext that generates unused extension bits,
+// converting it to a zext.
 //
 //===----------------------------------------------------------------------===//
 
@@ -19,6 +20,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/DemandedBits.h"
 #include "llvm/Analysis/GlobalsModRef.h"
+#include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/InitializePasses.h"
@@ -33,6 +35,8 @@ using namespace llvm;
 
 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
+STATISTIC(NumSExt2ZExt,
+          "Number of sign extension instructions converted to zero extension");
 
 /// If an instruction is trivialized (dead), then the chain of users of that
 /// instruction may need to be cleared of assumptions that can no longer be
@@ -109,6 +113,24 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
       continue;
     }
 
+    // Convert SExt into ZExt if none of the extension bits is required
+    if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
+      APInt Demanded = DB.getDemandedBits(SE);
+      const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
+      auto *const DstTy = SE->getDestTy();
+      const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
+      if (Demanded.countLeadingZeros() >= (DestBitSize - SrcBitSize)) {
+        clearAssumptionsOfUsers(SE, DB);
+        IRBuilder<> Builder(SE);
+        I.replaceAllUsesWith(
+            Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
+        Worklist.push_back(SE);
+        Changed = true;
+        NumSExt2ZExt++;
+        continue;
+      }
+    }
+
     for (Use &U : I.operands()) {
       // DemandedBits only detects dead integer uses.
       if (!U->getType()->isIntOrIntVectorTy())

llvm/test/Transforms/BDCE/sext_multi_uses.ll

@@ -1,11 +1,11 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -o - -bdce -S %s | FileCheck %s
+; RUN: opt -S -bdce < %s | FileCheck %s
 define i32 @ZEXT_0(i16 %a) {
 ; CHECK-LABEL: @ZEXT_0(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[EXT:%.*]] = sext i16 [[A:%.*]] to i32
-; CHECK-NEXT:    [[AND:%.*]] = and i32 [[EXT]], 65280
-; CHECK-NEXT:    [[LSR:%.*]] = lshr i32 [[EXT]], 8
+; CHECK-NEXT:    [[EXT1:%.*]] = zext i16 [[A:%.*]] to i32
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[EXT1]], 65280
+; CHECK-NEXT:    [[LSR:%.*]] = lshr i32 [[EXT1]], 8
 ; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[LSR]], 255
 ; CHECK-NEXT:    [[OR:%.*]] = or i32 [[AND]], [[AND2]]
 ; CHECK-NEXT:    ret i32 [[OR]]
@@ -22,10 +22,10 @@ entry:
 define i32 @ZEXT_1(i16 %a) {
 ; CHECK-LABEL: @ZEXT_1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[EXT:%.*]] = sext i16 [[A:%.*]] to i32
-; CHECK-NEXT:    [[LSR:%.*]] = lshr i32 [[EXT]], 8
+; CHECK-NEXT:    [[EXT1:%.*]] = zext i16 [[A:%.*]] to i32
+; CHECK-NEXT:    [[LSR:%.*]] = lshr i32 [[EXT1]], 8
 ; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[LSR]], 255
-; CHECK-NEXT:    [[AND:%.*]] = or i32 [[EXT]], -65536
+; CHECK-NEXT:    [[AND:%.*]] = or i32 [[EXT1]], -65536
 ; CHECK-NEXT:    [[OR:%.*]] = or i32 [[AND]], [[AND2]]
 ; CHECK-NEXT:    ret i32 [[OR]]
 ;
@@ -99,8 +99,8 @@ entry:
 
 define i16 @clear_assumptions(i8 %x, i16 %y) {
 ; CHECK-LABEL: @clear_assumptions(
-; CHECK-NEXT:    [[EXT:%.*]] = sext i8 [[X:%.*]] to i16
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i16 [[EXT]], [[Y:%.*]]
+; CHECK-NEXT:    [[EXT1:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[ADD:%.*]] = add i16 [[EXT1]], [[Y:%.*]]
 ; CHECK-NEXT:    [[AND:%.*]] = and i16 [[ADD]], 255
 ; CHECK-NEXT:    ret i16 [[AND]]
 ;