[PATCH] D36679: [InstCombine] Added support for: trunc(ashr(mul(sext(...), sext(...))) -> ashr(mul(...))

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: lib/Transforms/InstCombine/InstCombineShifts.cpp:562-565
     // (X >>u C) << C --> X & (-1 << C)
-    if (match(Op0, m_LShr(m_Value(X), m_Specific(Op1)))) {
+    // (X >>s C) << C --> X & (-1 << C)
+    if (match(Op0, m_LShr(m_Value(X), m_Specific(Op1))) ||
+        match(Op0, m_AShr(m_Value(X), m_Specific(Op1)))) {
----------------
Use 'm_Shr' to reduce the code. This change is independent of anything else, so I think it should go in first.
http://rise4fun.com/Alive/lBi
Here's a test you can use for that patch:

```
define i8 @shishi(i8 %x) {
  %a = ashr i8 %x, 6
  %b = shl i8 %a, 6
  %extra_use_of_a = mul i8 %a, 5
  %r = sdiv i8 %extra_use_of_a, %b
  ret i8 %r
}

```



https://reviews.llvm.org/D36679