When merging two basic blocks, it may happen that both of theses blocks contain a phi-source for the same phi-node. In this case, only the phi-source from the bottom BB must be taken in account, it kinda overwrites the value from the top BB and the phi-source from the top BB must be ignored, in fact it must be removed. However, it is not the case and this extra phi-source creates different kinds of problems. Among other things, it hinders further simplifications. For example, the following code: extern int array[2]; static inline int stupid_select(int idx) { if (idx) idx = 0; return array[idx]; } int select(void) { int d = stupid_select(-1); return d; } should boil down to a simple dereference of the array with an index of zero, like: select: load.32 %r8 <- 0[array] ret.32 %r8 but currently gives: select: phisrc.32 %phi3(idx) <- $0xffffffff phisrc.32 %phi4(idx) <- $0 phi.32 %r12(idx) <- %phi3(idx), %phi4(idx) sext.64 %r5 <- (32) %r12(idx) mul.64 %r6 <- %r5, $4 add.64 %r7 <- %r6, array load.32 %r8 <- 0[%r7] ret.32 %r8 This patch takes care of the problem by: * when merging 2 BBs, check when reaching a phi-source in the bottom BB * if one is found, look after sibling phi-sources * remove such sibling if belonging to the top BB. With this change, the code above gives: select: phisrc.32 %phi4(idx) <- $0 phi.32 %r12(idx) <- %phi4(idx) sext.64 %r5 <- (32) %r12(idx) mul.64 %r6 <- %r5, $4 add.64 %r7 <- %r6, array load.32 %r8 <- 0[%r7] ret.32 %r8 which can the be simplified into the expected result. Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>