[llvm-bugs] [Bug 27094] New: missed x.with.overflow optimizations

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27094

            Bug ID: 27094
           Summary: missed x.with.overflow optimizations
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: regehr at cs.utah.edu
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Here are a couple of examples where we should teach SCEV to see that the
overflows aren't going to happen.

regehr at hawking:~$ cat foo.c
void foo1(char *a) {
  for (int i = 0; i < 16; i++)
    a[i] = 0;
}

void foo2(char *a) {
  for (int i = 15; i >= 0; i--)
    a[i] = 0;
}
regehr at hawking:~$ clang -O2 -fsanitize=signed-integer-overflow
-fsanitize-trap=signed-integer-overflow foo.c -S -o - -emit-llvm
; ModuleID = 'foo.c'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind uwtable
define void @foo1(i8* nocapture %a) #0 {
entry:
  br label %for.body

for.cond.cleanup:                                 ; preds = %cont
  ret void

for.body:                                         ; preds = %entry, %cont
  %i.04 = phi i32 [ 0, %entry ], [ %2, %cont ]
  %idxprom = sext i32 %i.04 to i64
  %arrayidx = getelementptr inbounds i8, i8* %a, i64 %idxprom
  store i8 0, i8* %arrayidx, align 1, !tbaa !1
  %0 = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %i.04, i32 1)
  %1 = extractvalue { i32, i1 } %0, 1
  br i1 %1, label %trap, label %cont, !nosanitize !4

trap:                                             ; preds = %for.body
  tail call void @llvm.trap() #2, !nosanitize !4
  unreachable, !nosanitize !4

cont:                                             ; preds = %for.body
  %2 = extractvalue { i32, i1 } %0, 0
  %cmp = icmp slt i32 %2, 16
  br i1 %cmp, label %for.body, label %for.cond.cleanup
}

; Function Attrs: nounwind readnone
declare { i32, i1 } @llvm.sadd.with.overflow.i32(i32, i32) #1

; Function Attrs: noreturn nounwind
declare void @llvm.trap() #2

; Function Attrs: nounwind uwtable
define void @foo2(i8* nocapture %a) #0 {
entry:
  br label %for.body

for.cond.cleanup:                                 ; preds = %cont
  ret void

for.body:                                         ; preds = %entry, %cont
  %i.04 = phi i32 [ 15, %entry ], [ %2, %cont ]
  %idxprom = sext i32 %i.04 to i64
  %arrayidx = getelementptr inbounds i8, i8* %a, i64 %idxprom
  store i8 0, i8* %arrayidx, align 1, !tbaa !1
  %0 = tail call { i32, i1 } @llvm.ssub.with.overflow.i32(i32 %i.04, i32 1)
  %1 = extractvalue { i32, i1 } %0, 1
  br i1 %1, label %trap, label %cont, !nosanitize !4

trap:                                             ; preds = %for.body
  tail call void @llvm.trap() #2, !nosanitize !4
  unreachable, !nosanitize !4

cont:                                             ; preds = %for.body
  %2 = extractvalue { i32, i1 } %0, 0
  %cmp = icmp sgt i32 %2, -1
  br i1 %cmp, label %for.body, label %for.cond.cleanup
}

; Function Attrs: nounwind readnone
declare { i32, i1 } @llvm.ssub.with.overflow.i32(i32, i32) #1

attributes #0 = { nounwind uwtable "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = { nounwind readnone }
attributes #2 = { noreturn nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.9.0 (trunk 264413)"}
!1 = !{!2, !2, i64 0}
!2 = !{!"omnipotent char", !3, i64 0}
!3 = !{!"Simple C/C++ TBAA"}
!4 = !{}

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