When code size is the priority (Oz, MinSize attribute), help llvm

turning a code like this:

if (foo)
   free(foo)

into that:
free(foo)

Move a call to free from basic block FB into FB's predecessor, P,
when the path from P to FB is taken only if the argument of free is
not equal to NULL.

Some restrictions apply on P and FB to be sure that this code motion
is profitable. Namely:
1. FB must have only one predecessor P.
2. FB must contain only the call to free plus an unconditional
   branch to S.
3. P's successors are FB and S.

Because of 1., we will not increase the code size when moving the call
to free from FB to P.
Because of 2., FB will be empty after the move.
Because of 2. and 3., P's branch instruction becomes useless, so as FB
(simplifycfg will do the job).

llvm-svn: 171762

1 files changed, 29 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/malloc-free-delete.ll b/llvm/test/Transforms/InstCombine/malloc-free-delete.ll
index 4e3217dc2d9..cd12b29b118 100644
--- a/llvm/test/Transforms/InstCombine/malloc-free-delete.ll
+++ b/llvm/test/Transforms/InstCombine/malloc-free-delete.ll
@@ -91,3 +91,32 @@ define void @test5(i8* %ptr, i8** %esc) {
   store volatile i8 4, i8* %g
   ret void
 }
+
+;; When a basic block contains only a call to free and this block is accessed
+;; through a test of the argument of free against null, move the call in the
+;; predecessor block.
+;; Using simplifycfg will remove the empty basic block and the branch operation
+;; Then, performing a dead elimination will remove the comparison.
+;; This is what happens with -O1 and upper.
+; CHECK: @test6
+define void @test6(i8* %foo) minsize {
+; CHECK:  %tobool = icmp eq i8* %foo, null
+;; Call to free moved
+; CHECK-NEXT: tail call void @free(i8* %foo)
+; CHECK-NEXT: br i1 %tobool, label %if.end, label %if.then
+; CHECK: if.then:
+;; Block is now empty and may be simplified by simplifycfg
+; CHECK-NEXT:   br label %if.end
+; CHECK: if.end:
+; CHECK-NEXT:  ret void
+entry:
+  %tobool = icmp eq i8* %foo, null
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @free(i8* %foo)
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  ret void
+}