[llvm-bugs] [Bug 25476] New: __builtin_assume pessimization

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25476

            Bug ID: 25476
           Summary: __builtin_assume pessimization
           Product: libraries
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: matt at godbolt.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider the code: [c.f. https://goo.gl/7VB27y ]

--
enum class Side { Bid, Ask };
struct Foo { int a; int b; };

int test(Side side, const Foo &foo) {
  if (side == Side::Bid) return foo.a;
  return foo.b;
}

int test_with_unreachable(Side side, const Foo &foo) {
  if (side == Side::Bid) return foo.a;
  if (side == Side::Ask) return foo.b;
  __builtin_unreachable();
}

int test_with_assume(Side side, const Foo &foo) {
  __builtin_assume(side == Side::Bid || side == Side::Ask);
  if (side == Side::Bid) return foo.a;
  return foo.b;
}

int test_with_assume2(Side side, const Foo &foo) {
  if (side == Side::Bid) return foo.a;
  __builtin_assume(side == Side::Ask);
  return foo.b;
}
--

One might expect the generated code to basically be the same, but the _assume2
case generates a branch instead of the conditional moves emitted elsewhere.

e.g.
--
test(Side, Foo const&):                     # @test(Side, Foo const&)
    lea    rax, [rsi + 4]
    test    edi, edi
    cmove    rax, rsi
    mov    eax, dword ptr [rax]
    ret
--

vs

--
test_with_assume2(Side, Foo const&):       # @test_with_assume2(Side, Foo
const&)
    test    edi, edi
    je    .LBB3_2
    add    rsi, 4
.LBB3_2:
    mov    eax, dword ptr [rsi]
    ret
--

This would appear to be a pessimization.

Arguably given the __builtin_assumption that the Side is either 0 or 1, one
might hope the following code could be generated:

--
mov eax, dword ptr [rsi+edx*4]
ret
--

GCC has a similar bug with __builtin_unreachable
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68274)

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20151110/fecff595/attachment-0001.html>