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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54964 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README-SSE.txt b/lib/Target/X86/README-SSE.txt
index 87144248e3212e0daf50256dbdf59afa7b84136c..7c4bf37e34dbc32fd10e07cb2fc66245755fd5b4 100644 (file)
--- a/lib/Target/X86/README-SSE.txt
+++ b/lib/Target/X86/README-SSE.txt
@@ -839,3 +839,40 @@ _t:
        shufps  $2, %xmm1, %xmm2
        shufps  $132, %xmm2, %xmm0
        movaps  %xmm0, 0
+
+//===---------------------------------------------------------------------===//
+rdar://6037315
+
+llvm-gcc-4.2 does the following for uint32_t -> float conversions on i386:
+
+       uint32_t x;
+       float y = (float)x;
+
+becomes:
+
+movl   %eax,           -8(%ebp)        // write x to the stack
+movl   $0x3ff00000,    -4(%ebp)        // 2^52 + x as a double at -4(%ebp)
+movsd  -8(%ebp),               %xmm0
+subsd  [2^52 double],  %xmm0   // subtract 2^52 -- this is exact
+cvtsd2ss %xmm0,                %xmm0   // convert to single -- rounding happens here
+
+On merom/yonah, this takes a substantial stall.  The following is a much 
+better option:
+
+movd   %eax,           %xmm0   // load x into low word of xmm0
+movsd  [2^52 double],  %xmm1   // load 2^52 into xmm1
+orpd   %xmm1,          %xmm0   // 2^52 + x in double precision
+subsd  %xmm1,          %xmm0   // x in double precision
+cvtsd2ss %xmm0,                %xmm0   // x rounded to single precision
+
+IF we don't already need PIC, then the following is even faster still, at a 
+small cost to code size:
+
+movl           $0x3ff00000,    %ecx            // conjure high word of 2^52
+movd           %ecx,           %xmm1
+movss  %eax,           %xmm0   // load x into low word of xmm0
+psllq          $32,                    %xmm1   // 2^52
+orpd           %xmm1,          %xmm0   // 2^52 + x in double precision
+subsd          %xmm1,          %xmm0   // x in double precision
+cvtsd2ss       %xmm0,          %xmm0   // x in single precision
+