[llvm] r258422 - [InstCombine] Simplify (x >> y) <= x

[David Majnemer via llvm-commits]

Author: majnemer
Date: Thu Jan 21 12:55:54 2016
New Revision: 258422

URL: http://llvm.org/viewvc/llvm-project?rev=258422&view=rev
Log:
[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

Modified:
    llvm/trunk/lib/Analysis/InstructionSimplify.cpp
    llvm/trunk/test/Transforms/InstSimplify/compare.ll

Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=258422&r1=258421&r2=258422&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan 21 12:55:54 2016
@@ -2727,9 +2727,11 @@ static Value *SimplifyICmpInst(unsigned
     }
   }
 
+  // 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)

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=258422&r1=258421&r2=258422&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Thu Jan 21 12:55:54 2016
@@ -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