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I'm debugging an application that is running quite a bit slower when built as a 64-bit Linux ELF executable than as a 32-bit Linux ELF executable. Using Rational (IBM) Quantify, I tracked much of the performance difference down to (drum roll...) memset . Oddly, memset is taking a lot longer in the 64-bit executable. I am even able to see this with a small, simple application: #include <stdlib.h> #include <string.h> #define BUFFER_LENGTH 8000000 int main() { unsigned char* buffer = malloc(BUFFER_LENGTH * sizeof(unsigned char)); for(int i = 0; i < 10000; i++) memset(buffer, 0, BUFFER_LENGTH * sizeof(unsigned char)); } I build like this: $ gcc -m32 -std=gnu99 -g -O3 ms.c and $ gcc -m64 -std=gnu99 -g -O3 ms.c The wall-clock time as reported by time is longer for the -m64 build and Quantify confirms that the extra time is being spent in memset . So far I've tested in VirtualBox and VMWare (but not bare-metal Linux; I realize I need to do that next). The amount of extra time spent seems to vary a bit from one system to the next. What's going on here? Is there a well-known issue that my Google-foo is not able to uncover? EDIT: The disassembly ( gcc ... -S ) on my system shows that memset is being invoked as an external function: 32-bit: .LBB2: .loc 1 14 0 movl $8000000, 8(%esp) .loc 1 12 0 addl $1, %ebx .loc 1 14 0 movl $0, 4(%esp) movl %esi, (%esp) call memset 64-bit: .LBB2: .loc 1 14 0 xorl %esi, %esi movl $8000000, %edx movq %rbp, %rdi .LVL1: .loc 1 12 0 addl $1, %ebx .loc 1 14 0 call memset System: CentOS 5.7 2.6.18-274.17.1

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