Daniel Jasper 7e5f7305cf [InstCombine] Don't fold a GEP into itself through a PHI node ... This can only occur (I think) through the back-edge of the loop. However, folding a GEP into itself means that the value of the previous iteration needs to be stored in the meantime, thus requiring an additional register variable to be live, but not actually achieving anything (the gep still needs to be executed once per loop iteration). The attached test case is derived from: typedef unsigned uint32; typedef unsigned char uint8; inline uint8 *f(uint32 value, uint8 *target) { while (value >= 0x80) { value >>= 7; ++target; } ++target; return target; } uint8 *g(uint32 b, uint8 *target) { target = f(b, f(42, target)); return target; } What happens is that the GEP stored in incptr2 is folded into itself through the loop's back-edge and the phi-node stored in loopptr, effectively incrementing the ptr by "2" in each iteration instead of "1". In this case, it is actually increasing the number of GEPs required as the GEP before the loop can't be folded away anymore. For comparison: With this patch: define i8* @test4(i32 %value, i8* %buffer) { entry: %cmp = icmp ugt i32 %value, 127 br i1 %cmp, label %loop.header, label %exit loop.header: ; preds = %entry br label %loop.body loop.body: ; preds = %loop.body, %loop.header %buffer.pn = phi i8* [ %buffer, %loop.header ], [ %loopptr, %loop.body ] %newval = phi i32 [ %value, %loop.header ], [ %shr, %loop.body ] %loopptr = getelementptr inbounds i8, i8* %buffer.pn, i64 1 %shr = lshr i32 %newval, 7 %cmp2 = icmp ugt i32 %newval, 16383 br i1 %cmp2, label %loop.body, label %loop.exit loop.exit: ; preds = %loop.body br label %exit exit: ; preds = %loop.exit, %entry %0 = phi i8* [ %loopptr, %loop.exit ], [ %buffer, %entry ] %incptr3 = getelementptr inbounds i8, i8* %0, i64 2 ret i8* %incptr3 } Without this patch: define i8* @test4(i32 %value, i8* %buffer) { entry: %incptr = getelementptr inbounds i8, i8* %buffer, i64 1 %cmp = icmp ugt i32 %value, 127 br i1 %cmp, label %loop.header, label %exit loop.header: ; preds = %entry br label %loop.body loop.body: ; preds = %loop.body, %loop.header %0 = phi i8* [ %buffer, %loop.header ], [ %loopptr, %loop.body ] %loopptr = phi i8* [ %incptr, %loop.header ], [ %incptr2, %loop.body ] %newval = phi i32 [ %value, %loop.header ], [ %shr, %loop.body ] %shr = lshr i32 %newval, 7 %incptr2 = getelementptr inbounds i8, i8* %0, i64 2 %cmp2 = icmp ugt i32 %newval, 16383 br i1 %cmp2, label %loop.body, label %loop.exit loop.exit: ; preds = %loop.body br label %exit exit: ; preds = %loop.exit, %entry %ptr2 = phi i8* [ %incptr2, %loop.exit ], [ %incptr, %entry ] %incptr3 = getelementptr inbounds i8, i8* %ptr2, i64 1 ret i8* %incptr3 } Review: http://reviews.llvm.org/D8245 llvm-svn: 232718		2015-03-19 11:05:08 +00:00
..
Hello	[CMake] Add LLVM_LINK_COMPONENTS to loadable modules, LLVMHello and BugpointPasses, on Win32.	2014-07-13 13:36:48 +00:00
InstCombine	[InstCombine] Don't fold a GEP into itself through a PHI node	2015-03-19 11:05:08 +00:00
Instrumentation	[asan] remove redundant ifndefs. NFC	2015-03-17 19:13:23 +00:00
IPO	LowerBitSets: do not use private aliases at all on Darwin.	2015-03-16 23:36:24 +00:00
ObjCARC	One more try with unused.	2015-03-16 08:00:27 +00:00
Scalar	Fix GCC -Wparentheses warning (& reformat now that the precedence is fixed)	2015-03-17 17:48:24 +00:00
Utils	[ConstantRange] Split makeICmpRegion in two.	2015-03-18 00:41:24 +00:00
Vectorize	Try to fix a test broken by one of my previous commits.	2015-03-17 20:31:56 +00:00
CMakeLists.txt
LLVMBuild.txt
Makefile