[InstCombine] Simplify (x >> y) <= x

This commit extends the patterns recognised by InstSimplify to also handle (x >> y) <= x in the same way as (x /u y) <= x.

The missing optimisation was found investigating why LLVM did not optimise away bound checks in a binary search: https://github.com/rust-lang/rust/pull/30917

Patch by Andrea Canciani!

Differential Revision: http://reviews.llvm.org/D16402

llvm-svn: 258422

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2727,9 +2727,11 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // x >> y <=u x
   // x udiv y <=u x.
-  if (LBO && match(LBO, m_UDiv(m_Specific(RHS), m_Value()))) {
-    // icmp pred (X /u Y), X
+  if (LBO && (match(LBO, m_LShr(m_Specific(RHS), m_Value())) ||
+              match(LBO, m_UDiv(m_Specific(RHS), m_Value())))) {
+    // icmp pred (X op Y), X
     if (Pred == ICmpInst::ICMP_UGT)
       return getFalse(ITy);
     if (Pred == ICmpInst::ICMP_ULE)

llvm/test/Transforms/InstSimplify/compare.ll

@@ -397,6 +397,22 @@ define i1 @lshr3(i32 %x) {
 ; CHECK: ret i1 true
 }
 
+define i1 @lshr4(i32 %X, i32 %Y) {
+; CHECK-LABEL: @lshr4(
+  %A = lshr i32 %X, %Y
+  %C = icmp ule i32 %A, %X
+  ret i1 %C
+; CHECK: ret i1 true
+}
+
+define i1 @lshr5(i32 %X, i32 %Y) {
+; CHECK-LABEL: @lshr5(
+  %A = lshr i32 %X, %Y
+  %C = icmp ugt i32 %A, %X
+  ret i1 %C
+; CHECK: ret i1 false
+}
+
 define i1 @ashr1(i32 %x) {
 ; CHECK-LABEL: @ashr1(
   %s = ashr i32 -1, %x