For something like

uint32_t hi(uint64_t res)
{
        uint_32t hi = res >> 32;
        return !hi;
}

llvm IR looks like this:
define i32 @hi(i64 %res) nounwind uwtable ssp {
entry:
  %lnot = icmp ult i64 %res, 4294967296
  %lnot.ext = zext i1 %lnot to i32
  ret i32 %lnot.ext
}

The optimizer has optimize away the right shift and truncate but the resulting
constant is too large to fit in the 32-bit immediate field. The resulting x86
code is worse as a result:
        movabsq $4294967296, %rax       ## imm = 0x100000000
        cmpq    %rax, %rdi
        sbbl    %eax, %eax
        andl    $1, %eax

This patch teaches the x86 lowering code to handle ult against a large immediate
with trailing zeros. It will issue a right shift and a truncate followed by
a comparison against a shifted immediate.
        shrq    $32, %rdi
        testl   %edi, %edi
        sete    %al
        movzbl  %al, %eax

It also handles a ugt comparison against a large immediate with trailing bits
set. i.e. X >  0x0ffffffff -> (X >> 32) >= 1

rdar://11866926

llvm-svn: 160312

0 files changed, 0 insertions, 0 deletions