https://llvm.org/bugs/show_bug.cgi?id=31278
Bug ID: 31278
Summary: Inliner applies incorrect value mapping for recursive
calls
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: bjoern.boenningh...@tu-dortmund.de
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17713
--> https://llvm.org/bugs/attachment.cgi?id=17713&action=edit
code for simple demo reproducing incorrect inlining
"clang -O3" or "opt -inline" may work incorrectly on recursive functions. I
could narrow down and reproduce this for a simple function:
define void @tryme(i32* %ret, i32 %mode, i32 %val) #0 {
%1 = icmp ne i32 %mode, 0
br i1 %1, label %4, label %2
; <label>:2 ; preds = %0
%3 = add nsw i32 %val, 5
store i32 %3, i32* %ret, align 4
br label %6
; <label>:4 ; preds = %0
%5 = sub nsw i32 %val, 5
call void @tryme(i32* %ret, i32 0, i32 %5)
br label %6
; <label>:6 ; preds = %4, %2
ret void
}
This should store %val + 5 to %ret if %mode == 0, and %val otherwise.
opt -inline will transform this to:
define void @tryme(i32* %ret, i32 %mode, i32 %val) #0 {
%1 = icmp ne i32 %mode, 0
br i1 %1, label %4, label %2
; <label>:2 ; preds = %0
%3 = add nsw i32 %val, 5
store i32 %3, i32* %ret, align 4
br label %6
; <label>:4 ; preds = %0
%5 = sub nsw i32 %val, 5
store i32 %5, i32* %ret, align 4
br label %6
; <label>:6 ; preds = %4, %2
ret void
}
Notice how the second store now stores %5 = %val - 5 (instead of %val - 5 + 5 =
%val).
Attached you will find some C code that produces above and also breaks with
clang -O3, I assume for the same reason.
I tried to track down the root cause in the inliner, and got to the
per-instruction loop in the PruningFunctionCloner::CloneBlock in
lib/Transforms/Utils/CloneFunction.cpp. Here, %val in %3 of the called function
gets mapped to %5, then %3 is correctly simplified to %val of the caller. It
seems like, as caller and callee are identical, %val is now incorrectly mapped
back to %5, thus %3 in is replaced by %5, not %val. It appears to me as if
currently VMap content is not suitable to distinguish caller and callee
versions of Values and their respective mappings for recursive functions.
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