https://llvm.org/bugs/show_bug.cgi?id=26067
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: unassignedb...@nondot.org
Reporter: chisophu...@gmail.com
CC: llvm-bugs@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
retq
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
retq
.LBB1_3:
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]
retq
(cmp_eq is substantially improved from clang 3.6 btw. Play around here:
https://goo.gl/RVJeGa
)
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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