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[docs][nfc] Fix code-formatting in UB docs #139079

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May 8, 2025
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[docs][nfc] Fix code-formatting in UB docs #139079

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18 changes: 10 additions & 8 deletions llvm/docs/UndefinedBehavior.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -300,12 +300,12 @@ Take the following example function:
entry:
br label %loop

loop:
loop:
%i = phi i32 [ 0, %entry ], [ %i2, %loop.end ]
%cond = icmp ule i32 %i, %n
br i1 %cond, label %loop.cont, label %exit

loop.cont:
loop.cont:
br i1 %c, label %then, label %else

then:
Expand Down Expand Up @@ -334,22 +334,22 @@ We would obtain the following IR:
entry:
br i1 %c, label %then, label %else

then:
then:
%i = phi i32 [ 0, %entry ], [ %i2, %then.cont ]
%cond = icmp ule i32 %i, %n
br i1 %cond, label %then.cont, label %exit

then.cont:
then.cont:
...
%i2 = add i32 %i, 1
br label %then

else:
else:
%i3 = phi i32 [ 0, %entry ], [ %i4, %else.cont ]
%cond = icmp ule i32 %i3, %n
br i1 %cond, label %else.cont, label %exit

else.cont:
else.cont:
...
%i4 = add i32 %i3, 1
br label %else
Expand Down Expand Up @@ -381,6 +381,8 @@ We can make the loop unswitching optimization above correct as follows:
entry:
%c2 = freeze i1 %c
br i1 %c2, label %then, label %else
...
}


Writing Tests Without Undefined Behavior
Expand All @@ -399,7 +401,7 @@ particular divisor value because our optimization kicks in regardless:
define i32 @fn(i8 %a) {
%div = udiv i8 %a, poison
...
}
}

The issue with this test is that it triggers immediate UB. This prevents
verification tools like Alive from validating the correctness of the
Expand All @@ -412,7 +414,7 @@ The test above should use a dummy function argument instead of using poison:
define i32 @fn(i8 %a, i8 %dummy) {
%div = udiv i8 %a, %dummy
...
}
}

Common sources of immediate UB in tests include branching on undef/poison
conditions and dereferencing undef/poison/null pointers.
Expand Down
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