[LLVMbugs] [Bug 5822] New: StringRef::endswith compiles into gross code

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=5822

           Summary: StringRef::endswith compiles into gross code
           Product: libraries
           Version: 1.0
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: code-quality
          Severity: normal
          Priority: P2
         Component: Support Libraries
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: clattner at apple.com
                CC: llvmbugs at cs.uiuc.edu, daniel at zuster.org


This code:

#include "llvm/ADT/StringRef.h"
bool test(llvm::StringRef R) { return R.endswith("x"); }
bool test2(llvm::StringRef R) { return !R.empty() && R.back() == 'x'; }

compiles into:

define zeroext i8 @_Z4testN4llvm9StringRefE(i8* %R.0, i64 %R.1) nounwind
readonly ssp {
entry:
  %0 = add i64 %R.1, -1                           ; <i64> [#uses=2]
  %1 = icmp ult i64 %0, %R.1                      ; <i1> [#uses=1]
  %iftmp.50.0.i.i.i = select i1 %1, i64 %0, i64 %R.1 ; <i64> [#uses=4]
  %2 = icmp ugt i64 %iftmp.50.0.i.i.i, %R.1       ; <i1> [#uses=1]
  %iftmp.51.0.i.i.i = select i1 %2, i64 %iftmp.50.0.i.i.i, i64 %R.1 ; <i64>
[#uses=2]
  %3 = icmp ult i64 %iftmp.51.0.i.i.i, %R.1       ; <i1> [#uses=1]
  %iftmp.50.0.i5.i.i = select i1 %3, i64 %iftmp.51.0.i.i.i, i64 %R.1 ; <i64>
[#uses=1]
  %4 = sub i64 %iftmp.50.0.i5.i.i, %iftmp.50.0.i.i.i ; <i64> [#uses=1]
  %5 = icmp eq i64 %4, 1                          ; <i1> [#uses=1]
  br i1 %5, label %bb.i.i, label %_ZNK4llvm9StringRef8endswithES0_.exit

bb.i.i:                                           ; preds = %entry
  %6 = getelementptr inbounds i8* %R.0, i64 %iftmp.50.0.i.i.i ; <i8*> [#uses=1]
  %lhsv = load i8* %6                             ; <i8> [#uses=1]
  %7 = icmp eq i8 %lhsv, 120                      ; <i1> [#uses=1]
  %retval.i.i = zext i1 %7 to i8                  ; <i8> [#uses=1]
  ret i8 %retval.i.i

_ZNK4llvm9StringRef8endswithES0_.exit:            ; preds = %entry
  ret i8 0
}

which is pretty gross.  This is due to instcombine not zapping the max's, which
may be due to StringRef being written wrong (e.g. using unsigned instead of
signed integers).  GCC also generates nasty code.

This should compile down into:
  return !R.empty() && R.back() == 'x';

which codegen's to:

define zeroext i8 @_Z5test2N4llvm9StringRefE(i8* %R.0, i64 %R.1) ssp {
entry:
  %0 = icmp eq i64 %R.1, 0                        ; <i1> [#uses=1]
  br i1 %0, label %bb8, label %_ZNK4llvm9StringRef4backEv.exit

_ZNK4llvm9StringRef4backEv.exit:                  ; preds = %entry
  %1 = add i64 %R.1, -1                           ; <i64> [#uses=1]
  %2 = getelementptr inbounds i8* %R.0, i64 %1    ; <i8*> [#uses=1]
  %3 = load i8* %2, align 1                       ; <i8> [#uses=1]
  %4 = icmp eq i8 %3, 120                         ; <i1> [#uses=1]
  %retval = zext i1 %4 to i8                      ; <i8> [#uses=1]
  ret i8 %retval

bb8:                                              ; preds = %entry
  ret i8 0
}

In .s, this is the difference:

__Z4testN4llvm9StringRefE:
Leh_func_begin1:
        leaq    -1(%rsi), %rax
        movq    %rsi, %rcx
        cmpq    %rsi, %rax
        cmovae  %rsi, %rax
        cmpq    %rsi, %rax
        cmova   %rax, %rcx
        cmpq    %rsi, %rcx
        cmovae  %rsi, %rcx
        subq    %rax, %rcx
        cmpq    $1, %rcx
        jne     LBB1_2
        cmpb    $120, (%rdi,%rax)
        sete    %al
        movzbl  %al, %eax
        ret
LBB1_2:
        xorl    %eax, %eax
        ret

vs

__Z5test2N4llvm9StringRefE:
Leh_func_begin2:
        testq   %rsi, %rsi
        je      LBB2_2
        cmpb    $120, -1(%rsi,%rdi)
        sete    %al
        movzbl  %al, %eax
        ret
LBB2_2:
        xorl    %eax, %eax
        ret


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