[llvm] [clang-tools-extra] [flang] [lldb] [clang] [IndVars] Add check of loop invariant for trunc instructions (PR #71072)

[Markos Horro via cfe-commits]

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@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=indvars -S | FileCheck %s
+
+declare void @foo(i16 noundef)
+
+; Function Attrs: mustprogress noreturn uwtable
+define void @bar(i64 noundef %ptr) {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[PTR:%.*]] to i4
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i4 [[TMP0]] to i16
+; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
+; CHECK:       while.body:
+; CHECK-NEXT:    tail call void @foo(i16 noundef signext [[TMP1]])
+; CHECK-NEXT:    br label [[WHILE_BODY]]
+;
+entry:
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %while.body
+  %0 = phi i64 [ %ptr, %entry ], [ %add.ptr, %while.body ]
+  %1 = trunc i64 %0 to i16
+  %and = and i16 %1, 15                           ; loop invariant
+  tail call void @foo(i16 noundef signext %and)
----------------
markoshorro wrote:

Having something like:

`declare void @foo(i2 noundef)

entry:
  %ptr.addr.0 = ptrtoint ptr %ptr to i64
  br label %while.body

while.body:                                       ; preds = %entry, %while.body
  %0 = phi i64 [ %ptr.addr.0, %entry ], [ %add.ptr, %while.body ]
  %1 = trunc i64 %0 to i2
  %and = and i2 %1, 15                           ; loop invariant
  tail call void @foo(i2 noundef signext %and)
  %add.ptr = add nsw i64 %0,  16
  br label %while.body
}`

With this change, the IR generated is the following:

`declare void @foo(i2 noundef)

define void @bar(ptr noundef %ptr) {
entry:
  %ptr.addr.0 = ptrtoint ptr %ptr to i64
  %0 = trunc i64 %ptr.addr.0 to i2
  br label %while.body

while.body:                                       ; preds = %while.body, %entry
  %and = and i2 %0, -1
  tail call void @foo(i2 noundef signext %and)
  br label %while.body
}`

This is valid to me, when you say that `trunc` can affect the result, do you have any example in mind? Thanks!

https://github.com/llvm/llvm-project/pull/71072