[llvm-bugs] [Bug 26067] New: Unable to de-abstract std::tuple comparisons

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Jan 7 14:24:34 PST 2016


            Bug ID: 26067
           Summary: Unable to de-abstract std::tuple comparisons
           Product: libraries
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: chisophugis at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider this case, where lexicographical comparison of bit fields is
equivalent to a direct integer comparison of the `Data` field:

#include <stdint.h>
#include <tuple>

struct A {
  uint32_t Data;
  std::tuple<int, int, int> getAsTuple() const {
    return std::make_tuple((Data & 0xFFFF0000) >> 16, (Data & 0xFF00) >> 8,
(Data & 0xFF));

bool cmp_eq(A LHS, A RHS) {
  return LHS.getAsTuple() == RHS.getAsTuple();

bool cmp_lt(A LHS, A RHS) {
  return LHS.getAsTuple() < RHS.getAsTuple();

This produces the following assembly with clang 3.7:

cmp_eq(A, A):                          # @cmp_eq(A, A)
        cmpl    %edi, %esi
        sete    %al

cmp_lt(A, A):                          # @cmp_lt(A, A)
        movl    %esi, %ecx
        movl    %edi, %edx
        movl    %edx, %esi
        shrl    $16, %esi
        movl    %ecx, %edi
        shrl    $16, %edi
        movb    $1, %al
        cmpl    %edi, %esi
        jb      .LBB1_5
        cmpl    %esi, %edi
        jae     .LBB1_3
        xorl    %eax, %eax
        movzbl  %dh, %esi  # NOREX
        movzbl  %ch, %edi  # NOREX
        cmpl    %edi, %esi
        jb      .LBB1_5
        movzbl  %dl, %eax
        movzbl  %cl, %ecx
        cmpl    %ecx, %eax
        sbbb    %cl, %cl
        cmpl    %esi, %edi
        setae   %al
        andb    %cl, %al
.LBB1_5:                                # %bool std::operator< <int, int, int,
int, int, int>(std::tuple<int, int, int> const&, std::tuple<int, int, int>
const&) [clone .exit]

(cmp_eq is substantially improved from clang 3.6 btw. Play around here:

Clang 3.7 gets cmp_eq "right". But cmp_lt is far from optimal (it should be the
same as cmp_eq but choosing a different condition code). This pattern just
cropped up for me in the wild on some non-LLVM code that I was working on.

Using std::make_tuple/std::tie to synthesize lexicographical comparisons
dramatically simplifies code, so I expect this sort of situation with adjacent
bit fields to crop up in the wild as a byproduct (I don't know how much it has
caught on, but it likely will catch on if it hasn't). In LLVM, we are using
std::make_tuple/std::tie for lexicographical comparisons pretty extensively
(e.g. r202755).

At least in this case one can uglify the code to get the right codegen (just
manually compare the `Data` field), but in the bitfield case I don't think
there's even a well-defined way to get the right codegen https://goo.gl/sCLraB

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