[flang-commits] [flang] b812932 - [FLANG] Change loop versioning to use shift instead of divide

[Mats Petersson via flang-commits]

Author: Mats Petersson
Date: 2023-06-01T19:29:57+01:00
New Revision: b812932b3582752eb45240044be8ac1a9df4759f

URL: https://github.com/llvm/llvm-project/commit/b812932b3582752eb45240044be8ac1a9df4759f
DIFF: https://github.com/llvm/llvm-project/commit/b812932b3582752eb45240044be8ac1a9df4759f.diff

LOG: [FLANG] Change loop versioning to use shift instead of divide

Despite me being convinced that the use of divide didn't produce any
divide instructions, it does in fact add more instructions than using
a plain shift operation.

This patch simply changes the divide to a shift right, with an
assert to check that the "divisor" is a power of two.

Reviewed By: kiranchandramohan, tblah

Differential Revision: https://reviews.llvm.org/D151880

Added: 
    

Modified: 
    flang/lib/Optimizer/Transforms/LoopVersioning.cpp
    flang/test/Transforms/loop-versioning.fir

Removed: 
    


################################################################################
diff  --git a/flang/lib/Optimizer/Transforms/LoopVersioning.cpp b/flang/lib/Optimizer/Transforms/LoopVersioning.cpp
index f1588d2f622a1..92430d458948c 100644
--- a/flang/lib/Optimizer/Transforms/LoopVersioning.cpp
+++ b/flang/lib/Optimizer/Transforms/LoopVersioning.cpp
@@ -262,15 +262,19 @@ void LoopVersioningPass::runOnOperation() {
                                             loc, curIndex, totalIndex)
                                       : curIndex;
           }
-          mlir::Value elemSize =
-              builder.createIntegerConstant(loc, idxTy, arg.size);
           // This is the lowest dimension - which doesn't need scaling
           mlir::Value finalIndex =
               builder.createConvert(loc, idxTy, coop->getOperand(1));
           if (totalIndex) {
+            assert(llvm::isPowerOf2_32(arg.size) &&
+                   "Expected power of two here");
+            unsigned bits = llvm::Log2_32(arg.size);
+            mlir::Value elemShift =
+                builder.createIntegerConstant(loc, idxTy, bits);
             totalIndex = builder.create<mlir::arith::AddIOp>(
                 loc,
-                builder.create<mlir::arith::DivSIOp>(loc, totalIndex, elemSize),
+                builder.create<mlir::arith::ShRSIOp>(loc, totalIndex,
+                                                     elemShift),
                 finalIndex);
           } else {
             totalIndex = finalIndex;

diff  --git a/flang/test/Transforms/loop-versioning.fir b/flang/test/Transforms/loop-versioning.fir
index 3c8930ccbde47..6fc8eb852c1cf 100644
--- a/flang/test/Transforms/loop-versioning.fir
+++ b/flang/test/Transforms/loop-versioning.fir
@@ -366,9 +366,9 @@ func.func @sum1dfixed(%arg0: !fir.ref<!fir.array<?xf64>> {fir.bindc_name = "a"},
 // Check the 2D -> 1D coordinate conversion, should have a multiply and a final add.
 // Some other operations are checked to synch the 
diff erent parts.
 // CHECK: %[[OUTER_IDX:.*]] = arith.muli %[[DIMS1]]#2, {{.*}}
-// CHECK: %[[ITEMSIZE:.*]] = arith.constant 8 : index
 // CHECK: %[[INNER_IDX:.*]] = fir.convert {{.*}}
-// CHECK: %[[OUTER_DIV:.*]] = arith.divsi %[[OUTER_IDX]], %[[ITEMSIZE]]
+// CHECK: %[[ITEMSHIFT:.*]] = arith.constant 3 : index
+// CHECK: %[[OUTER_DIV:.*]] = arith.shrsi %[[OUTER_IDX]], %[[ITEMSHIFT]]
 // CHECK: %[[C2D:.*]] = arith.addi %[[OUTER_DIV]], %[[INNER_IDX]]
 // CHECK: %[[COORD:.*]] = fir.coordinate_of %[[BOXADDR]], %[[C2D]] : (!fir.ref<!fir.array<?xf64>>, index) -> !fir.ref<f64>
 // CHECK: %{{.*}} = fir.load %[[COORD]] : !fir.ref<f64>
@@ -498,9 +498,9 @@ func.func @sum1dfixed(%arg0: !fir.ref<!fir.array<?xf64>> {fir.bindc_name = "a"},
 // CHECK: %[[OUTER_IDX:.*]] = arith.muli %[[DIMS2]]#2, {{.*}}
 // CHECK: %[[MIDDLE_IDX:.*]] = arith.muli %[[DIMS1]]#2, {{.*}}
 // CHECK: %[[MIDDLE_SUM:.*]] = arith.addi %[[MIDDLE_IDX]], %[[OUTER_IDX]]
-// CHECK: %[[ITEMSIZE:.*]] = arith.constant 8 : index
 // CHECK: %[[INNER_IDX:.*]] = fir.convert {{.*}}
-// CHECK: %[[MIDDLE_DIV:.*]] = arith.divsi %[[MIDDLE_SUM]], %[[ITEMSIZE]]
+// CHECK: %[[ITEMSHIFT:.*]] = arith.constant 3 : index
+// CHECK: %[[MIDDLE_DIV:.*]] = arith.shrsi %[[MIDDLE_SUM]], %[[ITEMSHIFT]]
 // CHECK: %[[C3D:.*]] = arith.addi %[[MIDDLE_DIV]], %[[INNER_IDX]]
 // CHECK: %[[COORD:.*]] = fir.coordinate_of %[[BOXADDR]], %[[C3D]] : (!fir.ref<!fir.array<?xf64>>, index) -> !fir.ref<f64>
 // CHECK: %{{.*}} = fir.load %[[COORD]] : !fir.ref<f64>