https://llvm.org/bugs/show_bug.cgi?id=28880
Bug ID: 28880
Summary: GVNHoist leads to miscompile of llvm test-suite
SingleSource/Benchmarks/Misc/fbench.c
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: ma...@braunis.de
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
GVN-hoist appears to miscompile SingleSource/Benchmarks/Misc/fbench.c as this
builder:
http://lab.llvm.org:8080/green/job/Performance_O3flto/
is broken after r277786 "GVN-hoist: enable by default".
Citing Sebastians analysis: "
think there is a problem with the MemorySSA when compiling fbench with -Ofast
The problem appears with -mllvm -gvn-max-hoisted=63, (at 62 the
program is not miscompiled.)
before.ll is the MSSA dump at 62 and after.ll is the MSSA for 63.
We end up hoisting two loads past a store, and the loads do not link
to the store: they are both liveOnEntry:
; MemoryUse(liveOnEntry)
%19 = load double, double* @axis_slope_angle, align 8, !tbaa !5
and
; MemoryUse(liveOnEntry)
%22 = load double, double* @axis_slope_angle, align 8, !tbaa !5
past a store to the global variable:
; 7 = MemoryDef(liveOnEntry)
store double 0.000000e+00, double* @axis_slope_angle, align 8, !tbaa !5
"
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