https://llvm.org/bugs/show_bug.cgi?id=25031
Bug ID: 25031
Summary: PowerPC code generation bug: relative branch cannot be
resolved by linker
Product: new-bugs
Version: 3.7
Hardware: Other
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: eric.schwe...@pgroup.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 14988
--> https://llvm.org/bugs/attachment.cgi?id=14988&action=edit
a potential patch
There is an issue in the algorithm in PPCBranchSelector.cpp. The algorithm
pre-computes the size of each basic block in a function including any
additional padding needed between adjacent blocks due to alignment directives.
Unfortunately, this is a dynamic programming problem as the adjustment of
branches can alter the padding needed as the instruction stream is rewritten.
A small example:
MBB1: (say this is 16 byte aligned)
instr1
instr2
bcc ..., another_galaxy
MBB2:
bcc ..., another_solar_system
.align 4
MBB3: (alignment is specified 16 bytes)
Here we would precompute that MBB2 gets 0 bytes of padding. However, this must
be recomputed as after expanding the two distant branches, the .align 4 will
cause 8 bytes of padding, throwing off the precomputed size of MBB2 and any
branches that compute the distance using the size of MBB2. This can cause a
label to drift too far away.
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
