DAGCombiner: make sure or/shl/srl really has zero high bits before forming bswap

We want to convert code like (or (srl N, 8), (shl N, 8)) into (srl (bswap N),
const), but this is only valid if the bits above 16 on the source pattern are
0, the checks we were doing on this were slightly wrong before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189348 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 101 insertions, 6 deletions

diff --git a/test/CodeGen/X86/bswap.ll b/test/CodeGen/X86/bswap.ll
index 9e46592a4a..e6a456c39d 100644
--- a/test/CodeGen/X86/bswap.ll
+++ b/test/CodeGen/X86/bswap.ll
@@ -1,6 +1,7 @@
 ; bswap should be constant folded when it is passed a constant argument
 
 ; RUN: llc < %s -march=x86 -mcpu=i686 | FileCheck %s
+; RUN: llc < %s -march=x86-64 | FileCheck %s --check-prefix=CHECK64
 
 declare i16 @llvm.bswap.i16(i16)
 
@@ -11,6 +12,9 @@ declare i64 @llvm.bswap.i64(i64)
 define i16 @W(i16 %A) {
 ; CHECK-LABEL: W:
 ; CHECK: rolw $8, %ax
+
+; CHECK64-LABEL: W:
+; CHECK64: rolw $8, %
         %Z = call i16 @llvm.bswap.i16( i16 %A )         ; <i16> [#uses=1]
         ret i16 %Z
 }
@@ -18,6 +22,9 @@ define i16 @W(i16 %A) {
 define i32 @X(i32 %A) {
 ; CHECK-LABEL: X:
 ; CHECK: bswapl %eax
+
+; CHECK64-LABEL: X:
+; CHECK64: bswapl %
         %Z = call i32 @llvm.bswap.i32( i32 %A )         ; <i32> [#uses=1]
         ret i32 %Z
 }
@@ -26,6 +33,9 @@ define i64 @Y(i64 %A) {
 ; CHECK-LABEL: Y:
 ; CHECK: bswapl %eax
 ; CHECK: bswapl %edx
+
+; CHECK64-LABEL: Y:
+; CHECK64: bswapq %
         %Z = call i64 @llvm.bswap.i64( i64 %A )         ; <i64> [#uses=1]
         ret i64 %Z
 }
@@ -33,9 +43,13 @@ define i64 @Y(i64 %A) {
 ; rdar://9164521
 define i32 @test1(i32 %a) nounwind readnone {
 entry:
-; CHECK: test1
-; CHECK: bswapl %eax
-; CHECK: shrl $16, %eax
+; CHECK-LABEL: test1:
+; CHECK: bswapl [[REG:%.*]]
+; CHECK: shrl $16, [[REG]]
+
+; CHECK64-LABEL: test1:
+; CHECK64: bswapl [[REG:%.*]]
+; CHECK64: shrl $16, [[REG]]
   %and = lshr i32 %a, 8
   %shr3 = and i32 %and, 255
   %and2 = shl i32 %a, 8
@@ -46,9 +60,13 @@ entry:
 
 define i32 @test2(i32 %a) nounwind readnone {
 entry:
-; CHECK: test2
-; CHECK: bswapl %eax
-; CHECK: sarl $16, %eax
+; CHECK-LABEL: test2:
+; CHECK: bswapl [[REG:%.*]]
+; CHECK: sarl $16, [[REG]]
+
+; CHECK64-LABEL: test2:
+; CHECK64: bswapl [[REG:%.*]]
+; CHECK64: sarl $16, [[REG]]
   %and = lshr i32 %a, 8
   %shr4 = and i32 %and, 255
   %and2 = shl i32 %a, 8
@@ -57,3 +75,80 @@ entry:
   %conv3 = ashr exact i32 %sext, 16
   ret i32 %conv3
 }
+
+@var8 = global i8 0
+@var16 = global i16 0
+
+; The "shl" below can move bits into the high parts of the value, so the
+; operation is not a "bswap, shr" pair.
+
+; rdar://problem/14814049
+define i64 @not_bswap() {
+; CHECK-LABEL: not_bswap:
+; CHECK-NOT: bswapl
+; CHECK: ret
+
+; CHECK64-LABEL: not_bswap:
+; CHECK64-NOT: bswapq
+; CHECK64: ret
+  %init = load i16* @var16
+  %big = zext i16 %init to i64
+
+  %hishifted = lshr i64 %big, 8
+  %loshifted = shl i64 %big, 8
+
+  %notswapped = or i64 %hishifted, %loshifted
+
+  ret i64 %notswapped
+}
+
+; This time, the lshr (and subsequent or) is completely useless. While it's
+; technically correct to convert this into a "bswap, shr", it's suboptimal. A
+; simple shl works better.
+
+define i64 @not_useful_bswap() {
+; CHECK-LABEL: not_useful_bswap:
+; CHECK-NOT: bswapl
+; CHECK: ret
+
+; CHECK64-LABEL: not_useful_bswap:
+; CHECK64-NOT: bswapq
+; CHECK64: ret
+
+  %init = load i8* @var8
+  %big = zext i8 %init to i64
+
+  %hishifted = lshr i64 %big, 8
+  %loshifted = shl i64 %big, 8
+
+  %notswapped = or i64 %hishifted, %loshifted
+
+  ret i64 %notswapped
+}
+
+; Finally, it *is* OK to just mask off the shl if we know that the value is zero
+; beyond 16 bits anyway. This is a legitimate bswap.
+
+define i64 @finally_useful_bswap() {
+; CHECK-LABEL: finally_useful_bswap:
+; CHECK: bswapl [[REG:%.*]]
+; CHECK: shrl $16, [[REG]]
+; CHECK: ret
+
+; CHECK64-LABEL: finally_useful_bswap:
+; CHECK64: bswapq [[REG:%.*]]
+; CHECK64: shrq $48, [[REG]]
+; CHECK64: ret
+
+  %init = load i16* @var16
+  %big = zext i16 %init to i64
+
+  %hishifted = lshr i64 %big, 8
+  %lomasked = and i64 %big, 255
+  %loshifted = shl i64 %lomasked, 8
+
+  %swapped = or i64 %hishifted, %loshifted
+
+  ret i64 %swapped
+}
+