From 63f97201dc9dcebbe84d1b73113166c64212b4b8 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Fri, 17 Oct 2008 01:33:43 +0000 Subject: Fun x86 encoding tricks: when adding an immediate value of 128, use a SUB instruction instead of an ADD, because -128 can be encoded in an 8-bit signed immediate field, while +128 can't be. This avoids the need for a 32-bit immediate field in this case. A similar optimization applies to 64-bit adds with 0x80000000, with the 32-bit signed immediate field. To support this, teach tablegen how to handle 64-bit constants. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57663 91177308-0d34-0410-b5e6-96231b3b80d8 --- test/CodeGen/X86/add-trick32.ll | 11 +++++++++++ 1 file changed, 11 insertions(+) create mode 100644 test/CodeGen/X86/add-trick32.ll (limited to 'test/CodeGen/X86/add-trick32.ll') diff --git a/test/CodeGen/X86/add-trick32.ll b/test/CodeGen/X86/add-trick32.ll new file mode 100644 index 0000000..42909b4 --- /dev/null +++ b/test/CodeGen/X86/add-trick32.ll @@ -0,0 +1,11 @@ +; RUN: llvm-as < %s | llc -march=x86 > %t +; RUN: not grep add %t +; RUN: grep subl %t | count 1 + +; The immediate can be encoded in a smaller way if the +; instruction is a sub instead of an add. + +define i32 @foo(i32 inreg %a) nounwind { + %b = add i32 %a, 128 + ret i32 %b +} -- cgit v1.1