------- Comment #54 from mikulas at artax dot karlin dot mff dot cuni dot cz 2009-09-27 09:03 ------- (In reply to comment #51)
For 4.4, the designers held two wrong assumptions: 1) the incoming stack is always aligned on -mincoming-stack-boundary (wrong for functions called from assembler or code generated by other compilers). 2) all the variables must be aligned on their alignment (wrong for double, long double, long long, mmx: the processor may accept them misaligned). The assumption 1) generates crashing code (for example Seamonkey). The assumption 2) generates suboptimal code (bug #40667). The assumption 1) can be trivially quashed with parameter -mincoming-stack-boundary=2, then the code will be non-crashing, but you will be hitting problem 2) and the code will be ugly and slow: any function containing double or long double variable will generate code for stack realigning and a frame pointer. (for long long this inefficiency was partially fixed in 4.4.1, but only partially, single long long variables don't trigger the alignment after 4.4.1 but structures with long long do, see bug #40667). So: to fix problem 1), gcc should assume that the incoming stack is 4-byte aligned. To fix problem 2), instead of single alignment, types and variables should have two alignments: preferred alignment and enforced alignment (so that you don't realign the stack if there is double on it, but you realign it if there is 16-byte SSE). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
