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

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
Herald added a subscriber: mcrosier.

I noticed this missed optimization in the CGP memcmp() expansion, and then saw that we don't have the fold in InstCombine.

It wasn't immediately clear to me that a vector bswap swaps the bytes of each element in the vector while leaving the elements in place. Should I add a blurb about that in the LangRef?


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
@@ -2984,9 +2984,7 @@
 
 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)
@@ -2999,9 +2997,7 @@
 
 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)
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3437,6 +3437,10 @@
     }
   }
 
+  // If both operands are byte-swapped, just compare the original values.
+  if (match(Op0, m_BSwap(m_Value(A))) && match(Op1, m_BSwap(m_Value(B))))
+    return new ICmpInst(Pred, A, B);
+
   return nullptr;
 }
 


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