[PATCH] D34763: [InstCombine] look through bswaps for equality comparisons

[Sanjay Patel via Phabricator via llvm-commits]

spatel updated this revision to Diff 104496.
spatel added a comment.

Patch updated:

1. Added bitreverse transform since that's a close relative.
2. I made the constant operand matcher a TODO for now since I don't have evidence of it.

Interesting side note: we probably don't want to make this change in the memcmp expansion yet because we produce worse (sometimes substantially worse) code for both PPC and x86 with this transform. Ie, we fail to use a byte-reversing memory instruction, and/or we have more register usage and several extra instructions because of that.


https://reviews.llvm.org/D34763

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp.ll


Index: test/Transforms/InstCombine/icmp.ll
===================================================================
--- test/Transforms/InstCombine/icmp.ll
+++ test/Transforms/InstCombine/icmp.ll
@@ -2979,9 +2979,7 @@
 
 define i1 @bswap_ne(i32 %x, i32 %y) {
 ; CHECK-LABEL: @bswap_ne(
-; CHECK-NEXT:    [[SWAPX:%.*]] = call i32 @llvm.bswap.i32(i32 %x)
-; CHECK-NEXT:    [[SWAPY:%.*]] = call i32 @llvm.bswap.i32(i32 %y)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[SWAPX]], [[SWAPY]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 %x, %y
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %swapx = call i32 @llvm.bswap.i32(i32 %x)
@@ -2994,9 +2992,7 @@
 
 define <8 x i1> @bswap_vec_eq(<8 x i16> %x, <8 x i16> %y) {
 ; CHECK-LABEL: @bswap_vec_eq(
-; CHECK-NEXT:    [[SWAPX:%.*]] = call <8 x i16> @llvm.bswap.v8i16(<8 x i16> %x)
-; CHECK-NEXT:    [[SWAPY:%.*]] = call <8 x i16> @llvm.bswap.v8i16(<8 x i16> %y)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <8 x i16> [[SWAPX]], [[SWAPY]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <8 x i16> %x, %y
 ; CHECK-NEXT:    ret <8 x i1> [[CMP]]
 ;
   %swapx = call <8 x i16> @llvm.bswap.v8i16(<8 x i16> %x)
@@ -3009,9 +3005,7 @@
 
 define i1 @bitreverse_eq(i64 %x, i64 %y) {
 ; CHECK-LABEL: @bitreverse_eq(
-; CHECK-NEXT:    [[REVX:%.*]] = call i64 @llvm.bitreverse.i64(i64 %x)
-; CHECK-NEXT:    [[REVY:%.*]] = call i64 @llvm.bitreverse.i64(i64 %y)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i64 [[REVX]], [[REVY]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i64 %x, %y
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %revx = call i64 @llvm.bitreverse.i64(i64 %x)
@@ -3024,9 +3018,7 @@
 
 define <8 x i1> @bitreverse_vec_ne(<8 x i16> %x, <8 x i16> %y) {
 ; CHECK-LABEL: @bitreverse_vec_ne(
-; CHECK-NEXT:    [[REVX:%.*]] = call <8 x i16> @llvm.bitreverse.v8i16(<8 x i16> %x)
-; CHECK-NEXT:    [[REVY:%.*]] = call <8 x i16> @llvm.bitreverse.v8i16(<8 x i16> %y)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne <8 x i16> [[REVX]], [[REVY]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne <8 x i16> %x, %y
 ; CHECK-NEXT:    ret <8 x i1> [[CMP]]
 ;
   %revx = call <8 x i16> @llvm.bitreverse.v8i16(<8 x i16> %x)
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3437,6 +3437,15 @@
     }
   }
 
+  // If both operands are byte-swapped or bit-reversed, just compare the
+  // original values.
+  // TODO: If one operand is a constant, we can swap/reverse it and eliminate
+  // the use of the swapped/reversed variable.
+  if ((match(Op0, m_BSwap(m_Value(A))) && match(Op1, m_BSwap(m_Value(B)))) ||
+      (match(Op0, m_Intrinsic<Intrinsic::bitreverse>(m_Value(A))) &&
+       match(Op1, m_Intrinsic<Intrinsic::bitreverse>(m_Value(B)))))
+    return new ICmpInst(Pred, A, B);
+
   return nullptr;
 }
 


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