[PATCH] D59374: [InstCombine] canonicalize funnel shift constant shift amount to be modulo bitwidth

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: nikic, lebedev.ri, nlopes.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

The shift argument is defined to be modulo the bitwidth, so if that argument is a constant, we can always reduce the constant to its minimal form to allow better CSE and other follow-on transforms.

There was already a test that shows a follow-on simplification, so I assume we just had not made our way around to adding this transform yet.


https://reviews.llvm.org/D59374

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/fsh.ll


Index: llvm/test/Transforms/InstCombine/fsh.ll
===================================================================
--- llvm/test/Transforms/InstCombine/fsh.ll
+++ llvm/test/Transforms/InstCombine/fsh.ll
@@ -310,7 +310,7 @@
 
 define i32 @fshl_constant_shift_amount_modulo_bitwidth(i32 %x, i32 %y) {
 ; CHECK-LABEL: @fshl_constant_shift_amount_modulo_bitwidth(
-; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.fshl.i32(i32 [[X:%.*]], i32 [[Y:%.*]], i32 33)
+; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.fshl.i32(i32 [[X:%.*]], i32 [[Y:%.*]], i32 1)
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %r = call i32 @llvm.fshl.i32(i32 %x, i32 %y, i32 33)
@@ -319,7 +319,7 @@
 
 define i33 @fshr_constant_shift_amount_modulo_bitwidth(i33 %x, i33 %y) {
 ; CHECK-LABEL: @fshr_constant_shift_amount_modulo_bitwidth(
-; CHECK-NEXT:    [[R:%.*]] = call i33 @llvm.fshr.i33(i33 [[X:%.*]], i33 [[Y:%.*]], i33 34)
+; CHECK-NEXT:    [[R:%.*]] = call i33 @llvm.fshr.i33(i33 [[X:%.*]], i33 [[Y:%.*]], i33 1)
 ; CHECK-NEXT:    ret i33 [[R]]
 ;
   %r = call i33 @llvm.fshr.i33(i33 %x, i33 %y, i33 34)
@@ -328,7 +328,7 @@
 
 define <2 x i32> @fshr_constant_shift_amount_modulo_bitwidth_vec(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @fshr_constant_shift_amount_modulo_bitwidth_vec(
-; CHECK-NEXT:    [[R:%.*]] = call <2 x i32> @llvm.fshr.v2i32(<2 x i32> [[X:%.*]], <2 x i32> [[Y:%.*]], <2 x i32> <i32 34, i32 -1>)
+; CHECK-NEXT:    [[R:%.*]] = call <2 x i32> @llvm.fshr.v2i32(<2 x i32> [[X:%.*]], <2 x i32> [[Y:%.*]], <2 x i32> <i32 2, i32 31>)
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
   %r = call <2 x i32> @llvm.fshr.v2i32(<2 x i32> %x, <2 x i32> %y, <2 x i32> <i32 34, i32 -1>)
@@ -337,17 +337,18 @@
 
 define <2 x i31> @fshl_constant_shift_amount_modulo_bitwidth_vec(<2 x i31> %x, <2 x i31> %y) {
 ; CHECK-LABEL: @fshl_constant_shift_amount_modulo_bitwidth_vec(
-; CHECK-NEXT:    [[R:%.*]] = call <2 x i31> @llvm.fshl.v2i31(<2 x i31> [[X:%.*]], <2 x i31> [[Y:%.*]], <2 x i31> <i31 34, i31 -1>)
+; CHECK-NEXT:    [[R:%.*]] = call <2 x i31> @llvm.fshl.v2i31(<2 x i31> [[X:%.*]], <2 x i31> [[Y:%.*]], <2 x i31> <i31 3, i31 1>)
 ; CHECK-NEXT:    ret <2 x i31> [[R]]
 ;
   %r = call <2 x i31> @llvm.fshl.v2i31(<2 x i31> %x, <2 x i31> %y, <2 x i31> <i31 34, i31 -1>)
   ret <2 x i31> %r
 }
 
-; The shift modulo bitwidth is the same for all vector elements, but this is not simplified yet.
+; The shift modulo bitwidth is the same for all vector elements.
+
 define <2 x i31> @fshl_only_op1_demanded_vec_nonsplat(<2 x i31> %x, <2 x i31> %y) {
 ; CHECK-LABEL: @fshl_only_op1_demanded_vec_nonsplat(
-; CHECK-NEXT:    [[Z:%.*]] = call <2 x i31> @llvm.fshl.v2i31(<2 x i31> [[X:%.*]], <2 x i31> [[Y:%.*]], <2 x i31> <i31 7, i31 38>)
+; CHECK-NEXT:    [[Z:%.*]] = lshr <2 x i31> [[Y:%.*]], <i31 24, i31 24>
 ; CHECK-NEXT:    [[R:%.*]] = and <2 x i31> [[Z]], <i31 63, i31 31>
 ; CHECK-NEXT:    ret <2 x i31> [[R]]
 ;
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1994,6 +1994,18 @@
 
   case Intrinsic::fshl:
   case Intrinsic::fshr: {
+    // Canonicalize a shift amount constant operand to be modulo the bit-width.
+    unsigned BitWidth = II->getType()->getScalarSizeInBits();
+    Constant *ShAmtC;
+    if (match(II->getArgOperand(2), m_Constant(ShAmtC))) {
+      Constant *WidthC = ConstantInt::get(II->getType(), BitWidth);
+      Constant *ModuloC = ConstantExpr::get(Instruction::URem, ShAmtC, WidthC);
+      if (ModuloC != ShAmtC) {
+        II->setArgOperand(2, ModuloC);
+        return II;
+      }
+    }
+
     const APInt *SA;
     if (match(II->getArgOperand(2), m_APInt(SA))) {
       Value *Op0 = II->getArgOperand(0), *Op1 = II->getArgOperand(1);
@@ -2020,7 +2032,6 @@
     // The shift amount (operand 2) of a funnel shift is modulo the bitwidth,
     // so only the low bits of the shift amount are demanded if the bitwidth is
     // a power-of-2.
-    unsigned BitWidth = II->getType()->getScalarSizeInBits();
     if (!isPowerOf2_32(BitWidth))
       break;
     APInt Op2Demanded = APInt::getLowBitsSet(BitWidth, Log2_32_Ceil(BitWidth));


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D59374.190672.patch
Type: text/x-patch
Size: 4224 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190314/733229ab/attachment.bin>