fix rdar://8494845 + PR8244 - a miscompile exposed by my patch in r101350

llvm-svn: 115294

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -4087,6 +4087,15 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
         if ((N0.getValueType().getSizeInBits() & (EVTBits-1)) != 0)
           return SDValue();
       }
+      
+      // If the shift amount is larger than the input type then we're not
+      // accessing any of the loaded bytes.  If the load was a zextload/extload
+      // then the result of the shift+trunc is zero/undef (handled elsewhere).
+      // If the load was a sextload then the result is a splat of the sign bit
+      // of the extended byte.  This is not worth optimizing for.
+      if (ShAmt >= VT.getSizeInBits())
+        return SDValue();
+      
     }
   }
 

llvm/test/CodeGen/X86/shift-folding.ll

@@ -1,21 +1,21 @@
 ; RUN: llc < %s -march=x86 | \
 ; RUN:   grep {s\[ah\]\[rl\]l} | count 1
 
-define i32* @test1(i32* %P, i32 %X) {
+define i32* @test1(i32* %P, i32 %X) nounwind {
         %Y = lshr i32 %X, 2             ; <i32> [#uses=1]
         %gep.upgrd.1 = zext i32 %Y to i64               ; <i64> [#uses=1]
         %P2 = getelementptr i32* %P, i64 %gep.upgrd.1           ; <i32*> [#uses=1]
         ret i32* %P2
 }
 
-define i32* @test2(i32* %P, i32 %X) {
+define i32* @test2(i32* %P, i32 %X) nounwind {
         %Y = shl i32 %X, 2              ; <i32> [#uses=1]
         %gep.upgrd.2 = zext i32 %Y to i64               ; <i64> [#uses=1]
         %P2 = getelementptr i32* %P, i64 %gep.upgrd.2           ; <i32*> [#uses=1]
         ret i32* %P2
 }
 
-define i32* @test3(i32* %P, i32 %X) {
+define i32* @test3(i32* %P, i32 %X) nounwind {
         %Y = ashr i32 %X, 2             ; <i32> [#uses=1]
         %P2 = getelementptr i32* %P, i32 %Y             ; <i32*> [#uses=1]
         ret i32* %P2

llvm/test/CodeGen/X86/store-narrow.ll

@@ -152,3 +152,17 @@ define void @test9() nounwind {
   store i32 %or, i32* @g_16
   ret void
 }
+
+; rdar://8494845 + PR8244
+; X64: test10:
+; X64-NEXT: movsbl	(%rdi), %eax
+; X64-NEXT: shrl	$8, %eax
+; X64-NEXT: ret
+define i8 @test10(i8* %P) nounwind ssp {
+entry:
+  %tmp = load i8* %P, align 1
+  %conv = sext i8 %tmp to i32
+  %shr3 = lshr i32 %conv, 8
+  %conv2 = trunc i32 %shr3 to i8
+  ret i8 %conv2
+}