[LLVMbugs] [Bug 18007] New: X86 backend epic fail at generating decent code for some obvious bit math

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18007

            Bug ID: 18007
           Summary: X86 backend epic fail at generating decent code for
                    some obvious bit math
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: chandlerc at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This is kinda sad.

% cat x.cpp
unsigned g(unsigned i, bool b1, bool b2, bool b3) {
  unsigned cookie_mask = ~(7 << 29);
  return ((i & cookie_mask) | (b1 << 31) | (b2 << 30) | (b3 << 29));
}

% ./bin/clang++ -O2 -S -emit-llvm -o - x.cpp
; ModuleID = 'x.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind readnone uwtable
define i32 @_Z1gjbbb(i32 %i, i1 zeroext %b1, i1 zeroext %b2, i1 zeroext %b3) #0
{
entry:
  %and = and i32 %i, 536870911
  %conv = zext i1 %b1 to i32
  %shl = shl nuw i32 %conv, 31
  %or = or i32 %shl, %and
  %conv4 = zext i1 %b2 to i32
  %shl5 = shl nuw nsw i32 %conv4, 30
  %or6 = or i32 %or, %shl5
  %conv8 = zext i1 %b3 to i32
  %shl9 = shl nuw nsw i32 %conv8, 29
  %or10 = or i32 %or6, %shl9
  ret i32 %or10
}

attributes #0 = { nounwind readnone uwtable "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
"unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"clang version 3.4 "}

Totally reasonable. Exactly what I would expect. nuw and nsw all over the
place, nothing ambiguous, etc.

% ./bin/clang++ -O2 -S -o - x.cpp
        .file   "x.cpp"
        .text
        .globl  _Z1gjbbb
        .align  16, 0x90
        .type   _Z1gjbbb, at function
_Z1gjbbb:                               # @_Z1gjbbb
        .cfi_startproc
# BB#0:                                 # %entry
        andl    $536870911, %edi        # imm = 0x1FFFFFFF
        movzbl  %sil, %eax
        shll    $31, %eax
        orl     %edi, %eax
        movzbl  %dl, %edx
        shll    $30, %edx
        orl     %eax, %edx
        movzbl  %cl, %eax
        shll    $29, %eax
        orl     %edx, %eax
        ret
.Ltmp1:
        .size   _Z1gjbbb, .Ltmp1-_Z1gjbbb
        .cfi_endproc

What on earth is this???

movzbl? We're about to shift out the high bits. Copying between registers? WAT

-- 
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/20131120/c52e50c5/attachment.html>