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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - combine multiple checks of __builtin_*_with_overflow()"
href="http://llvm.org/bugs/show_bug.cgi?id=16424">16424</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>combine multiple checks of __builtin_*_with_overflow()
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nunoplopes@sapo.pt
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>At least the x86 backend could be improved to combine checks from multiple
__builtin_*_with_overflow() calls.
E.g.:
target datalayout =
"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S128"
target triple = "i386-pc-linux-gnu"
; Function Attrs: nounwind readnone
define i32 @safe_address(i32 %nmemb, i32 %size, i32 %offset) {
entry:
%0 = tail call { i32, i1 } @llvm.umul.with.overflow.i32(i32 %nmemb, i32
%size)
%1 = extractvalue { i32, i1 } %0, 1
%2 = extractvalue { i32, i1 } %0, 0
%3 = tail call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 %2, i32 %offset)
%4 = extractvalue { i32, i1 } %3, 1
%or3 = or i1 %1, %4
br i1 %or3, label %return, label %if.end, !prof !0
if.end: ; preds = %entry
%5 = extractvalue { i32, i1 } %3, 0
br label %return
return: ; preds = %entry, %if.end
%retval.0 = phi i32 [ %5, %if.end ], [ 0, %entry ]
ret i32 %retval.0
}
!0 = metadata !{metadata !"branch_weights", i32 4, i32 64}
declare { i32, i1 } @llvm.umul.with.overflow.i32(i32, i32) #1
declare { i32, i1 } @llvm.uadd.with.overflow.i32(i32, i32) #1
compiles to:
safe_address: # @safe_address
# BB#0: # %entry
pushl %esi
movl 8(%esp), %eax
mull 12(%esp)
pushfl
popl %esi
addl 16(%esp), %eax
setb %dl
xorl %ecx, %ecx
pushl %esi
popfl
jo .LBB0_3
# BB#1: # %entry
testb %dl, %dl
jne .LBB0_3
# BB#2: # %if.end
movl %eax, %ecx
.LBB0_3: # %return
movl %ecx, %eax
popl %esi
ret
Although a simpler sequence (w/ just 1 branch) like the following could be used
instead:
movl 8(%esp), %eax
mull 12(%esp) # overflow in %edx
addl 16(%esp), %eax
adcl %edx, %edx
testl %edx, %edx
jne .LBB0_1 # jump if %edx + %edx + carry != 0
.LBB0_2:
# result already in EAX
ret
.LBB0_1:
xorl %eax, %eax
jmp .LBB0_2</pre>
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