https://bugs.llvm.org/show_bug.cgi?id=52040
Bug ID: 52040
Summary: compiler crash when optimizing 3 or more HADD/HSUB
intrinsics
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Created attachment 25315
--> https://bugs.llvm.org/attachment.cgi?id=25315&action=edit
Compiler output with error log/callstack
A week ago, I reported https://bugs.llvm.org/show_bug.cgi?id=51974 which was
fixed in commit 468ff703e114599ce8fb7457bd3c7ef0b219e952. However, I came
across a variant of it when at least 3 HSUB/HADD intrinsics are chained
together in a particular way. This causes a cycle in the DAG as well, even with
the new fix.
The following code is a minimal repro for it, causing an assert about cycles in
Debug and looping forever in Release:
#include <immintrin.h>
__m128i do_stuff(__m128i I0) {
__m128i A = _mm_hadd_epi16(I0, I0);
__m128i B = _mm_hadd_epi16(A, A);
__m128i C = _mm_hadd_epi16(B, A);
return C;
}
when compiled with 'clang -O1 -mavx2'. The same issue occurs if all the
_mm_hadd_epi16 calls are changed to one of _mm_hadd_epi32, _mm_hsub_epi16, or
_mm_hsub_epi32.
I looked into the code in combineVectorHADDSUB() in X86ISelLowering.cpp, and
noticed the following code being hit:
SDValue LHS0 = LHS.getOperand(0);
SDValue RHS0 = LHS.getOperand(1);
SDValue LHS1 = RHS.getOperand(0);
SDValue RHS1 = RHS.getOperand(1);
This appears to be a typo, and changing it to
SDValue LHS0 = LHS.getOperand(0);
SDValue LHS1 = LHS.getOperand(1);
SDValue RHS0 = RHS.getOperand(0);
SDValue RHS1 = RHS.getOperand(1);
avoids the DAG cycle, and seems to optimize the function correctly. I don't
know what knock-on effects this would have.
I have also attached the debug output from the compiler I get from the test
case above which shows the cycle in the DAG and a callstack of the assert.
I tested on the latest trunk (commit e420164f40a907643db40e65fff51a6041d40090)
and it's still present. I have observed it on both Windows and Linux.
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