http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53438
Bug #: 53438 Summary: [4.7 Regression] Bitfield store replaced with full-byte store Classification: Unclassified Product: gcc Version: 4.7.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: wschm...@gcc.gnu.org Host: powerpc64-linux Target: powerpc64-linux Build: powerpc64-linux Created attachment 27461 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27461 Reduced testcase In the attached reduced testcase YarrPattern.ii, function void JSC::Yarr::YarrPatternConstructor::atomParenthesesSubpatternBegin(bool) (struct YarrPatternConstructor * const this, bool capture), the "capture" argument is copied into a one-bit bitfield m_capture within an inlined constructor. In 4.6, we get the proper read-modify-write code for the bitfield update. In 4.7, the value of capture is stored as a full byte instead. In the 077t.cplxlower dump, we see: <bb 8>: D.4581_14 = this_1(D)->m_alternative; D.3875.type = 7; D.3875.m_capture = capture_5(D); D.3875.m_invert = 0; ... In 078t.sra: <bb 8>: D.4581_14 = this_1(D)->m_alternative; SR.171_48 = 7; SR.172_47 = capture_5(D); SR.173_46 = 0; D.6531_55 = &MEM[(struct Vector *)D.4581_14].impl; MEM[(struct PatternTerm *)&t] = SR.171_48; MEM[(struct PatternTerm *)&t + 4B] = SR.172_47; t.m_invert = SR.173_46; ... In 079t.copyrename3: <bb 8>: D.4581_14 = this_1(D)->m_alternative; SR.171_48 = 7; capture_47 = capture_5(D); SR.173_46 = 0; D.6531_55 = &MEM[(struct Vector *)D.4581_14].impl; MEM[(struct PatternTerm *)&t] = SR.171_48; MEM[(struct PatternTerm *)&t + 4B] = capture_47; t.m_invert = SR.173_46; ... In 149t.optimized: <bb 9>: D.4581_14 = this_1(D)->m_alternative; D.6531_55 = &MEM[(struct Vector *)D.4581_14].impl; MEM[(struct PatternTerm *)&t] = 7; MEM[(struct PatternTerm *)&t + 4B] = capture_5(D); t.m_invert = 0; ... The bitfield-ness of m_capture is lost along the way. On powerpc64-linux, this generates a store-byte instruction instead of a read-modify-write. The reduced test doesn't show the correct code for 4.6 since the method in question is discarded for whatever reason. If needed, that can be seen in the larger YarrPattern.orig.ii, also attached. The regression remains in current trunk as well. Sorry for the relatively large test, but delta couldn't reduce it further and it didn't seem worthwhile to keep hacking on it by hand.