https://bugs.llvm.org/show_bug.cgi?id=43880
Bug ID: 43880
Summary: -expandmemcmp generates loads with incorrect alignment
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: juneyoung....@sf.snu.ac.kr
CC: llvm-bugs@lists.llvm.org
--
$ cat memcmp.ll
target datalayout = "e-i8:8:8-i16:16:16"
target triple = "x86_64-unknown-unknown"
declare i32 @memcmp(i8* nocapture, i8* nocapture, i64)
define i32 @cmp2(i8* nocapture readonly %x, i8* nocapture readonly %y) {
%call = tail call i32 @memcmp(i8* %x, i8* %y, i64 2)
ret i32 %call
}
$ ./llvm/bin/opt -expandmemcmp -S -o - memcmp.ll
; ModuleID = 'memcmp.ll'
source_filename = "memcmp.ll"
target datalayout = "e-i8:8:8-i16:16:16"
target triple = "x86_64-unknown-unknown"
declare i32 @memcmp(i8* nocapture, i8* nocapture, i64)
define i32 @cmp2(i8* nocapture readonly %x, i8* nocapture readonly %y) {
%1 = bitcast i8* %x to i16*
%2 = bitcast i8* %y to i16*
%3 = load i16, i16* %1
%4 = load i16, i16* %2
%5 = call i16 @llvm.bswap.i16(i16 %3)
%6 = call i16 @llvm.bswap.i16(i16 %4)
%7 = zext i16 %5 to i32
%8 = zext i16 %6 to i32
%9 = sub i32 %7, %8
ret i32 %9
}
; Function Attrs: nounwind readnone speculatable willreturn
declare i16 @llvm.bswap.i16(i16) #0
attributes #0 = { nounwind readnone speculatable willreturn }
--
This is incorrect because the loads in the output has align 2 (omitted
alignment in loads mean they have the ABI alignment). If %x or %y is not 2-byte
aligned, the optimized code raises undefined behavior, where as the source
wouldn't.
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