[llvm-bugs] [Bug 38590] New: Clang doesn't use add with carry when adding constants

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38590

            Bug ID: 38590
           Summary: Clang doesn't use add with carry when adding constants
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: ilya.lesokhin at gmail.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20729
  --> https://bugs.llvm.org/attachment.cgi?id=20729&action=edit
source file

When adding a register or a memory operand clang is able to use add with carry
to generate more efficient code.
See for example add_with_memory or add_with_register.

However, when adding a constant this optimization is missed.
See add_with_small_constants and add_with_large_constants

https://godbolt.org/g/2gb1ow

add_with_register():
 add    %rcx,%rdi
 adc    %r10,%rsi
 adc    %r9,%rdx
 adc    %r8,%rax
 retq   
 nopl   (%rax)
add_with_memory():
 add    0x200b09(%rip),%rcx        # 601030 <mem>
 adc    0x200b0a(%rip),%rdx        # 601038 <mem+0x8>
 adc    0x200b0b(%rip),%rsi        # 601040 <mem+0x10>
 adc    0x200b0c(%rip),%rax        # 601048 <mem+0x18>
 retq   
 nopl   (%rax)
add_with_small_constants():
 add    $0x1,%rcx
 adc    $0x0,%rdx
 setb   %al
 movzbl %al,%esi
 add    $0x2,%rdx
 adc    %rdi,%rsi
 setb   %al
 movzbl %al,%eax
 add    $0x3,%rsi
 adc    %r8,%rax
 add    $0x4,%rax
 retq   
 nopw   0x0(%rax,%rax,1)
add_with_large_constants_constant():
 movabs $0x3c208c16d87cfd47,%rdi
 add    %rax,%rdi
 adc    $0x0,%rcx
 setb   %al
 movzbl %al,%eax
 movabs $0x97816a916871ca8d,%rsi
 add    %rcx,%rsi
 adc    %rdx,%rax
 setb   %cl
 movzbl %cl,%ecx
 movabs $0xb85045b68181585d,%rdx
 add    %rax,%rdx
 adc    %r8,%rcx
 movabs $0x30644e72e131a029,%rax
 add    %rcx,%rax
 retq   
 nopl   0x0(%rax,%rax,1)

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